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/ 
LESSONS 



6r?> 



PHYSICAL GEOGRAPHY 



BY 

CHARLES R. DRYER, M.A., F.G.S.A. 

PROFESSOR OF GEOGRAPHY, INDIANA STATE NORMAL SCHOOL 




NEW YORK •:• CINCINNATI-: -CHICAGO 

AMERICAN BOOK COMPANY 



THE LIBRARY OF 
CONGRESS, 

Two Copies Received 

SEP. 6 1901 

n COPVRIGHT ENTRY 

C CLASS 0- XXc N». 

copy a. 



Copyright, 1901, by 
CHARLES R. DRYER. 

Entered at Stationers' Hall, London. 



DR. PHYS. GEOG. 
\V. P. I. 



PREFACE 

This book has been written in the belief that physical 
geography is not only interesting and valuable in its sub- 
ject matter, but is capable, when properly presented, of 
developing a scientific habit of mind. No attempt is made 
to discuss all the physical features of the earth, or those 
of any special region. The best type forms are selected 
and treated with sufficient fullness to give a clear and 
definite picture. From a study of the type general laws 
are developed, and the student is thus provided with a 
key for the solution of geographical problems wherever 
they may arise. 

Each topic is treated inductively. The essential facts 
are first given, and the student is then guided to a 
knowledge of their causes, significance, and results. This 
plan makes it possible to avoid in some degree the vague 
generalizations which are characteristic of most text-books. 
The student is in possession of a sufficient number of facts 
to enable him to see the basis and appreciate the value 
of the generalizations which follow. A large number of 
realistic exercises are introduced which appeal to the actual 
or possible experience of the student. They are designed 
not for the purpose of discovery but of realization, and to 
give some idea of the methods and possibilities of geo- 
graphic research. These exercises include both field and 
laboratory work. 

No pains has been spared to make the book scientifi- 
cally accurate and representative of the state of geographic 

5 



6 PREFACE 

science at the opening of the twentieth century. Due 
prominence is given to recent developments, but not to 
the exclusion of any link in the chain which connects the 
face of the earth with man. Discussions of topics which 
have a special bearing upon human interests are intro- 
duced at intervals throughout the book, and the relations 
of the physical features of the earth to human progress 
are systematically treated in a final chapter. 

The book has been written with a view to the needs of 
the teacher as well as to those of the student. Each topic 
is treated with sufficient fullness to enable the teacher to 
see its relation to other topics and to teach it intelligently. 
An unusually large number of illustrations have been 
selected with a view only to their teaching value. Appen- 
dixes give full instructions as to where good material and 
appliances for teaching may be obtained and how to use 
them. A bibliography of nearly all the geographical lit- 
erature available in English is added for the use of stu- 
dents, teachers, and those wishing to provide a good 
working library of the subject. 

In the arrangement of topics the logical order of the 
science is modified by the pedagogical order of presenta- 
tion to students. As a rule a topic is introduced where it 
is most needed in teaching ; but in many cases the order 
may be modified to suit individual or local conditions with- 
out inconvenience. Difficulties have not been avoided, 
but constant effort has been made to find the best way of 
overcoming them. The best method of learning is also 
the best method of teaching, and it is the hope of the 
author that this book may prove to be a substantial help 
in both. 

Terre Haute, Indiana. 



CONTENTS 



BOOK I. THE PLANET EARTH 



I. The Earth in Space . 
II. The Structure of the Earth 
III. The Face of the Earth 



9 
26 

38 



BOOK IE THE LAND 

IV. Erosion ...... 

V. The Mississippi River System 

VI. The Colorado River System 

VII. The St. Lawrence River System 

VIII. Underground Waters .... 

IX. Glaciers ...... 

X. The Drift Sheet of North America 

XI. Lakes and Lake Basins 

XII. The Development of Drainage Systems 

XIII. Forms of Sedimentation 

XIV. Mountains 

XV. Volcanoes . . ' . 

XVI. Land Sculpture ..... 

XVIL Coast Forms ..... 

XVIII. The Physiographic Cycle and the Classification of 

Land Forms ........ 



57 

68 

81 

92 

102 

108 

122 

135 
152 
168 

178 
194 
210 
227 

239 



BOOK III. THE SEA 



XIX. The Figure of the Sea . . . . . 243 

XX. Sea Water . . . . . . ... .250 

XXI. Movements of the Sea . . ' . . . . 258 

7 



CONTENTS 



BOOK IV. THE ATMOSPHERE 



XXII. 


The Air 






273 


XXIII. 


Moisture in the Air . . . 






280 


XXIV. 


Winds 






287 


XXV. 


Insolation and Temperature 






293 


XXVI. 


The Distribution of Pressure and Winds 






3 QI 


XXVII. 


Storms ...... 






312 


XXVIII. 


Rainfall ...... 






327 


XXIX. 


Weather and Climate .... 






335 



BOOK V. LIFE 

XXX. Plant Geography 

XXXI. Animal Geography ...... 

XXXII. The Geography of Man 

Appendix I. The Equipment of a Geographical Laboratory 

Appendix II. Meteorological Instruments 

Appendix III. The Construction of a Weather Map . 

Appendix IV. Reference Books , 



Index 



349 
364 

333 

393 
39S 
410 
412 

421 



LIST OF IMPORTANT MAPS 



Heights of the Land and Depths 

of the Sea. . . 40, 41 

Glacial and Champlain Periods, 

North America . . -125 

Glaciated Region of Europe . 133 
Distribution of Volcanoes . 208 

Mean Annual Surface Temper- 
ature of the Ocean . . 253 
Density of the Surface Water, 

and Surface Currents 256, 257 
Isotherms .... 295 

Temperature Zones . . . 298 



Annual Range of Average 

Monthly Temperature . 299 
Isobars and Winds . . . 303 
Ocean Winds . . . . 305 
Polar Whirls . . . 308, 309 

Rainfall . . . 32S, 330, 331 

Isotherms in United States . 344 
Absolute Annual Range of Tem- 
perature in United States . 345 
Rainfall in United States . . 345 

Vegetation Regions . . . 358 
Animal Realms and Regions . 367 



BOOK I. THE PLANET EARTH 

Alone, unpiloted, unswerving aye, 

The blind old earth spins on its trackless way. 

CHAPTER I 
THE EARTH IN SPACE 

The Earth appears to be Flat. — One who lives in a 
moderately level country sees the earth around him as a 
flat disk which stretches away in every direction to a circu- 
lar rim or horizon. Above the disk he sees the heavens 
or sky, like a dome or inverted bowl, with its edge resting 
all around on the rim of the earth. Every day he sees the 
sun rise above the edge of the earth on one side, pass over 
the arch of the sky, and disappear below the edge on the 
other side. At night the moon appears to follow a similar 
path, and the sky is studded all over with myriads of 
bright points. Sometimes clouds float between earth and 
sky and hide sun, -moon, and stars. In a hilly country it 
is necessary only to climb up to a high point to observe 
the same appearances, except that the earth disk seems 
larger and the sky dome more lofty. 

From any point of view it is evident also that objects in 
the distance look smaller than similar ones near by. To 
one looking along a railroad track, the telegraph poles 
appear to grow shorter and the rails nearer together, until 
they meet on the horizon. The same is true of the trees 
and houses on a long street. A horse or a man a mile 
away looks less than half his real size. Any object, how- 

9 



IO THE PLANET EARTH 

ever large, even a mountain, if far enough away, may ap- 
pear very small or be indistinguishable. On a bright, 
clear day distant objects look larger and nearer than on 
a dark day. A burning brush pile at night can be seen 
much farther than the unlighted pile by day. So the 
appearance of distant objects varies with their size and 
with the quantity of light that comes from them. 

Realistic Exercises. — Clear and definite ideas of distance can be 
acquired only by experience and practice. The student should give him- 
self a thorough training with yardstick and tapeline. Let him measure 
his book, the desk, schoolroom, building, yard, width of the street, city 
square, etc., not neglecting to measure heights as well as horizontal 
distances. He should practice estimating distances by the eye and then 
correct his estimates by measuring. A very convenient way of meas- 
uring is by the step or pace. Walk a hundred steps and measure the 
distance : do this repeatedly until the ability is acquired to step" a known 
and uniform space. In a short time any one can learn to measure dis- 
tances quite accurately by counting steps. Let one half of a class arrange 
themselves in line at regular intervals of one hundred yards, while the 
other half observe the apparent distance and size of the individuals. 

The Curvature of the Earth. — More than two thousand 
years ago scientific observers discovered that the surface of 
the earth is not flat, as it seems, but curved. When viewed 
from the water level on the shore of the ocean or of any body 
of water several miles across, the lower part of a ship or a 
tree, house, or other object at the same level near the oppo- 
site shore is hidden behind 
the curve of the water sur- 
face. At a distance of one 
mile the object is hidden to 
the height of eight inches, at 
two miles four times eight 
inches, at three miles nine 
times eight inches, and so on, according to the square of 
the number of miles. If the observer ascends a hill or 




THE EARTH IN SPACE 



II 



building, he can see farther over the curve; that is, the 
circle of the horizon enlarges. This is true also upon land. 
On a plane surface the horizon would be at an indefinite 
distance and would not retreat as the observer ascends. 

At places east of us the sun rises and sets earlier, and at 
places west of us later, than where we are. At Philadel- 
phia the sun rises an hour before it does at St. Louis, which 
is about eight hundred miles farther west. If the surface 
of the earth were flat, the sun would rise at all places at the 
same moment and set at the same moment. If the earth's 
surface were a plane, sun time would be the same at all 
places. The sun is so far away that it would appear at the 
same angle from Phila- 
delphia, St. Louis, and 
Denver, and if it were 
noon at one of these 
places, it would be noon 
at the others. Vertical 
lines at the three places 
would be parallel. But 
when it is noon at St. 
Louis it is i p. m. at 
Philadelphia and 1 1 a.m. at Denver. Vertical lines at the 
three places converge downward. Therefore the surface 
of the earth along an east and west line is curved. 

At a place where the sun is directly overhead at noon an 
upright post casts no shadow. ■ At places to the north or 
south an upright post at noon casts a shadow, the length 
of which becomes greater as the distance from the place 
of no shadow increases. If the earth's surface were a 
plane, the sun at noon, on account of its great distance, 
would appear everywhere at the same angle, and an upright 
post would cast a shadow of the same length everywhere. 



( 


ps 




< 


ps 




0S 


12 


M. 


PLANE 




M. 


EARTH 




M. 


DEN 


VER 




ST.L 


OUIS 




PHILADE 


LPHIA 




12 



THE PLANET EARTH 



To one who travels north or south, stars previously 
invisible rise in front of him, while stars behind him dis- 
appear below the horizon. The plane of the horizon tilts 

as he goes, sinking be- 
fore and rising behind, 
which is just what must 
occur on a curved sur- 
face. Thus a person at 
A (Fig. 3) can see the 
star x, but not y and z, 
while a person at C can 
the surface of the earth 




Fig. 3- 



see all three of the stars. Hence 
along a north and south line is curved. 

The shadow of the earth cast upon the moon during an 
eclipse always has a circular outline, on whatever side of 
the earth the moon happens to be. Only a spherical body 
casts a circular shadow in every position. 

Lastly, the form and size of the earth have been accu- 
rately measured by various methods. The mean equatorial 
diameter has been found to be 7926 miles, the polar diam- 
eter 7900 miles, and the mean circumference 24,860 miles. 

Realistic Exercises. — Ascend to the highest point you can reach, the 
tower or roof of a building, or the top of a tree or hill. How does the 
horizon change as you go up ? If possible, observe distant objects 
across a level stretch of country or a large body of water. If you 
travel several hundred miles east or west, how do you have to change 
your watch to make it agree with the time of the places you visit ? 

Raise a lamp or candle slowly from the floor to the level of a table : 
does its light strike all the objects upon the table at the same moment ? 
Lower it : does its light disappear from all parts of the surface of the 
table at the same moment ? 

If there is a hill in your vicinity, even a small one, stand at the foot 
of it and note the stars just visible above its top. Ascend the hill and 
notice how those stars rise higher above it and new stars come in sight. 
Descend upon the opposite side and notice how the stars disappear 



THE EARTH IN SPACE I 3 

behind the hill. Other objects, as trees and buildings, may be observed 
instead of stars. The curve of the hill produces in a small space the 
same effect as the curve of the earth. 

Consult the almanac for the time when an eclipse of the moon will 
occur, and observe the circular shadow of the earth. Hold a ball in 
various positions between a lamp and a wall : if the line from lamp to 
ball is perpendicular to the wall, what is the shape of the shadow ? 
Substitute various other objects for the ball, and compare results. 

The Starry Heavens. — If we observe the position of the 
conspicuous group of stars in the northern sky called the 
Great Dipper, 1 and then look for it again a few hours later, 
we shall find that, while the stars still form the outline of 
a clipper, the whole group has changed its position. 

By repeated observations we can map out in the heavens 
the path along which it is traveling and learn its speed, so 
that we can predict about where it will be in another hour 
or two. In this way, from a few evenings' watching, it 
will be plain that the stars in the northern sky appear to 
wheel around a central 
point, in a direction op- 
posite to the motion of 
the hands of a clock 
(counterclockwise), once 
in about twenty - four 
hours. The central point 
is near a not very bright 
star standing alone in a 
direct line with the outer Fig 4 - 

two stars of the Dipper, which are called the Pointers be- 
cause they always point toward the central star, called 

1 The Dipper is near the horizon due north, Sept. 22, at midnight; Oct. 23, at 10 
P.M.; Nov. 7, at 9 P.M.; Nov.' 22, at 8 P.M.; Dec. 7, at 7 P.M. In the northeast, 
Dec. 22, at midnight ; Jan. 20, at 10 P.M. ; Feb. 4, at 9 P.M. ; Feb. 19, at 8 P.M. On 
the meridian above the Polestar, March 21, at midnight; April 20, at 10 P.M.; 
May 5, at 9 p.m. ; May 21, at 8 p.m. 







* 




1 




* 


* 


+ 


+■ 


* > 








+ m • 






.* 


* 


POLARIS ^ 


* ' 








y' 


\ <, 








x 




* 


'' • 




*.._ 


• * 


t' 1 







14 THE PLANET EARTH 

Polaris or the Polestar. The circumpolar stars inside the 
circle described by the Dipper never set in our part of 
the world. 

If we watch some conspicuous group of stars like Orion, 
which in the autumn months rises early in the evening 1 

% , directly in the east, we shall 

pieces see fr s l° wr y mount the sky, 

it * pass a little south of the 

* zenith, and, about twelve 

^ *" bull hours after its rising, set 

* directly in the west. The 

other stars outside the circle 

* of the Dipper will be found 

i to rise somewhere on the 

*" eastern horizon, follow a 

0RI0N longer or shorter path across 

Fig 5- 

the heavens, and set some- 
where on the western horizon. The whole starry heavens 
seem to be turning about an axis tvhich passes through the 
Polestar, just as an open umbrella may be turned about its 
stick. If bits of paper are gummed to the inside of the um- 
brella, and it is held close to the head and turned, the papers 
will perform the same kind of a motion as the northern 
stars. If we had a large hollow globe with stars fastened 
all over its inner surface, and could place ourselves at the 
center of it while it rotates around us, it would imitate 
the apparent motion of all the stars, and we could see the 
whole path of each. The great hollow globe which seems 
to carry the stars, and at the center of which our home on 
the earth seems to be placed, is called the celestial sphere. 

1 Orion is just rising due east, Sept. 22, at midnight ; Oct. 23, at 10 P.M. ; Nov. 7, 
at 9 P.M. ; Nov. 22, at 8 P.M. ; Dec. 7, at 7 P.M. On the meridian, south of the zenith, 
Dec. 22, at 11 P.M. ; Jan. 20, at 9 p.m. ; Feb. 4, at 8 P.M.; Feb. 19, at 7 P.M. 



THE EARTH IN SPACE 



15 



The Apparent Path of the Sun. — The sun appears to 
revolve with the celestial sphere, but its path changes a 
little from day to day. On March 21 * (Vernal Equinox) it 
rises due east, passes south of the zenith at noon, and sets 
due west. After March 21 it rises farther and farther north 
of east, and sets farther and farther north of west until June 
2 1, 1 when it shines into our north windows for some time 

\V4 







Fig. 6. (From Todd's New Astronomy.) 

morning and evening, though it still passes south of our 
zenith at noon. Then the path of the sun begins to move 
back southward. On September 23 * it is just where it was 
on March 21, and on December 22 1 it rises farthest to the 
south of east, and sets farthest to the south of west. 

Realistic Exercises. — Choose some convenient place where the view 
is as little obstructed as possible. An open field is best, but an upper 
room or tower or roof of a building is good. From this spot deter- 
mine the four cardinal points on the horizon : the point directly below 
1 These dates vary a day or two in different years and centuries. 



i6 



THE PLANET EARTH 



57.3 Inches 



the sun at noon is south, the point directly below the Polestar is north. 
and the sunrise and sunset points on September 23 and March 21 are 
east and west respectively. Always standing in the same place, observe 
once every week or ten days the point on the horizon where the sun 
rises or sets or both ; also its distance above the south point at noon. 
Valuable observations may be made of the sunset point from a west 
room with several windows. Mark upon the floor a spot which com- 
mands as wide a range of the western horizon as possible. Observe 
from it the movement of the sunset point north or south from week 
to week. The extreme points reached on June 21 and December 22, 
and the middle point on September 23 and March 21 are especially im- 
portant, and their direction should be marked upon the window sill. 

The height of the sun at noon may be advantageously observed 
from a south window. Mark upon the floor from week to week the 

point reached by the shadow 
of the window sill at noon, or 
measure its distance from the 
wall and record it. with the date. 
This work may be done more 
accurately by the use of a de- 
vice for measuring angles. Find 
a building or fence the west 
side of which is in a north- 
south line. This may be de- 
termined at night by observing 
whether it is in line with the 
Polestar. At a point near the 
south end, and about six feet 
from the ground, drive a long nail horizontally so that it will project 
several inches. With a carpenter's leveling board, or with a square and 
plumbline, draw a horizontal line at the level of the nail, locate upon it 
a point 57.3 inches north of the nail, and mark it with a tack (Fig. 7). 
Fasten to the nail one end of a wire or a string which does not stretch, 
and with a pencil draw an arc downward from the tack to a point below 
the nail. This will be a quadrant of a circle three hundred and sixty 
inches in circumference, and the degrees, each one inch long, should 
be marked and numbered from the tack downward, and each degree 
divided into fourths or arcs of 15'. About noon, at the moment when 
the sun first shines upon the west side of the building, note the point 



N 




Fig 7- 



heliotrope. 



THE EARTH IN SPACE 17 

where the shadow of the nail falls across the quadrant. The angle 
between the shadow and the horizontal line will be the altitude of the 
sun, or its angular distance above the south point of the horizon. The 
angle between the shadow and the vertical line will be the angular dis- 
tance of the sun from the zenith. The altitude should be read and 
recorded once a week or ten days for at least six months. 

Rotation of the Earth. — For thousands of years even 
the wisest men believed the earth to be the center of 
the universe, and the sun, moon, and stars actually to 
revolve around it as they appear to do. ' The sun was 
supposed to be a small hot ball a few miles distant, and the 
stars to be only bright points ; therefore the velocity with 
which they must travel to complete a revolution around 
the earth every twenty-four hours did not seem so enor- 
mous to the ancients as it would to us. During the six- 
teenth and seventeenth centuries, the discoveries of Kepler, 
Galileo, and Newton 1 proved that the earth-center {geo- 
centric) theory of the universe is erroneous, and that the 
heavenly bodies appear to revolve around the earth from 
east to west because the earth is rotating from west to 
east upon an axis, the north end of which points toward 
the Polestar. The earth makes one complete rotation upon 
its axis in 23 h. 56 m. 4.09 s. 

Realistic Exercises. — The proof that the apparent motions of the' 
heavenly bodies are due to the rotation and revolution of the earth be- 
longs to astronomy rather than to geography, but these motions may be 
realized in several ways. If we watch the sunset closely, as the face of 
the sun appears to sink below the horizon, it is easy to see how in real- 
ity the horizon is rising between us and the sun. Again, as the face of 
the full moon appears to rise gradually above the horizon it is easy to 
see that the appearance is due to a sinking horizon. That is, as we 
watch the sun, moon, and stars rise, march from east to west, and set, 
we are really seeing the earth rotate from west to east. 

When riding rapidly upon a railroad train notice how objects in the 
landscape appear to be rushing by in the opposite direction. When 
1 Read about these men in the encyclopedia. 
DR. PHYS. GEOG. — 2 



THE PLANET EARTH 



sitting in a car, can you always tell whether a car upon the next track is 
moving one way, or your own car the other way ? 

Revolution of the Earth. — The earth travels along a 
nearly circular path through space at an average distance 
of nearly 93,000,000 miles from the sun. The circumfer- 
ence of this orbit is 584,600,000 miles. The earth revolves 
once around the sun (from one Vernal Equinox to the next) 
in 365 d. 5 h. 48 m. 46 s. About how many miles does it travel 
in its orbit each second ? The earth's orbit is an ellipse 
having the sun at one of the foci. • On January 2, the earth 
is nearly 91,500,000 miles from the sun, and on July 3, nearly 
94,500,000 miles. The diameter of the sun is about 880,000 
miles, or no times the earth's diameter, and the mean 
radius of the earth's orbit is 105 times the sun's diameter. 
Realistic Exercises. — In a room having an unobstructed floor space 
sixteen feet in diameter drive a small nail into the floor in the middle of 

the space. On each side of it 
in a north-south line drive an- 
other nail at a distance of an 
inch and a half. These nails 
should project an inch or two 
above the floor. Take a string 
which does not stretch easily, 
double it, and tie the ends so 
as to make a loop 94! inches 
long. Place one end of this 
loop around the two nails, and 
with a pencil at the other end 
draw upon the floor an ellipse 
having the nails for its foci. 
Around the north nail draw a 
circle eight ninths of an inch 
in diameter. The circle represents the sun, and the ellipse the orbit of 
the earth in correct proportion, the scale being one inch to a million 
miles. A proportional earth would be .008 inch in;diarheter. Mark 
the orbit with a line of ink or paint. 

Mark upon the ground a similar figure, using a doubled cord 94I feet 




Fig. 8. — How to draw an ellipse. 



THE EARTH IN SPACE 



19 



long (one foot to a million miles) . On this scale the sun will be nearly 
ten inches in diameter and the earth nearly one tenth of an inch. To 
realize the motion of the earth among the stars, walk around this path 
counterclockwise, with your face turned toward the center, (the sun), 
and notice how the distant objects beyond and on a line with the center 
continually change. Walk around again with your face turned directly 
away from the center (the sun), and notice a similar procession of 
objects in the line of sight passing backwards or clockwise. On account 
of the brightness of the sunlight we can not see the stars in a line with 
the sun, but at midnight we can look directly overhead and see the stars 
which are on the opposite side of the earth from the sun. If we thus 
observed the zenith at midnight every night for one year, we should see 
a procession of stars apparently passing westward, and at the end of the 
year the same stars would be in the zenith at midnight as at the begin- 
ning of the year. It is not necessary to make these observations every 
night or at midnight. If we observe the position of any star or group 
of stars, as Orion, at the same hour, say 9 p.m., once a week or ten days, 
it will be found in a position farther to the west at each successive 
observation. Thus in watching the apparent annual westward march 
of the stars, we are really seeing the eastward movement of the earth 
around the sun. 

NOON 




Fig. 9. 



The Change of Seasons. — That the sun's rays have 
greater heating power at noon than at morning or evening 
is one of the most familiar facts in nature. This is due 
chiefly to two causes, shown in Fig. 9. At sunrise and 
sunset, the rays, being horizontal, pass through a greater 
thickness of air, which absorbs more of their energy, and 
they are spread over more surface, so that there is less 



20 THE PLANET EARTH 

heat to the square mile. At noon, the rays, being more 
nearly vertical, pass through less air and cover less space, 
which makes the heat more intense. 1 

If the varying path of the sun in the heavens has been 
observed through the year (see p. 15), the facts are also 
familiar that in summer its path is longer and approaches 
nearer to the zenith than in winter. In June, the sun not 
only shines more directly in our latitude than in January, 
but also shines several hours longer each day. In spring 
and autumn, the angle of the rays and the length of 
daytime are intermediate. These changes are sufficient to 
account for the changing seasons. The causes of the 
variation in the path of the sun remain to be explained. 

The Attitude of the Earth. — As the earth moves around 
the sun, its axis always points toward the Polestar, and is 
always inclined at an angle of about 66^° to the plane 
passing through the earth's orbit and the center of the sun, 
or about 23^° from a perpendicular to that plane. Thus 
the earth presents at different times of the year different 
faces to the sun, as is shown in Fig. 10. 

Realistic Exercise. — It is not always easy to get a clear understand- 
ing of this subject from words and pictures, and resort should always be 
had to demonstration with some kind of apparatus, perhaps the simpler 
the better. Darken the room in which the orbit of the earth is marked 
upon the floor, place a lamp in the position of the sun, and carry a globe 
(almost any kind of a ball will answer) around the orbit counterclock- 
wise, holding it in such a position that the axis stands at an angle of 
66i° with the floor (the plane of the orbit), and the north end of it 
points directly north of the zenith. Observe that when the globe is in 
a position north of the lamp the northern hemisphere leans away from 
the lamp, the center of the lighted half is south of the equator, and the 
northern edge of the lighted half falls short of the north pole. When 

1 The idea sometimes met with that slanting rays heat less then direct rays be- 
cause they strike with less force, is erroneous. Rays of heat and light have no force 
of impact and do not strike at all in the same sense as a ball or an arrow does. 



THE EARTH IN SPACE 



21 




Fig. 10. — Position of the northern hemisphere throughout the year. 

the globe is south of the lamp, the northern hemisphere leans toward 
the lamp, the center of the lighted half is north of the equator, and its 
northern edge reaches beyond the north pole. When the globe is east 
or west of the lamp, the northern hemisphere is still inclined, but 
neither toward nor away from the lamp, the center of the lighted half 
is at the equator, and its edge just reaches either pole. Observe that at 
the center of the lighted half the rays strike the surface of the globe 
perpendicularly, at other places more and more slantingly as the dis- 
tance from the center increases, and at the edge of the lighted half the 
rays just graze the surface horizontally, or are tangent to it. 

Results of the Earth's Attitude. — If the axis were perpendicular to 
the plane of the orbit, that plane would always be the same as the plane 



22 THE PLANET EARTH 

of the equator, and the sun would always be vertically above some point 
on the equator. As it is, the sun is vertical at the equator at two oppo- 
site points in the earth's orbit, on September 23 and March 21. On 
June 21 the northern hemisphere is inclined toward the sun so that 
the vertical rays fall on the Tropic of Cancer, 23I north of the equator, 
and the tangent rays reach the Arctic Circle 23J- beyond the north pole. 
On December 22 the northern hemisphere is inclined away from the 
sun so that the vertical rays fall on the Tropic of Capricorn, 23^° south 
of the equator, and the tangent rays reach the Arctic Circle 23^° short 
of the north pole. Where do the tangent rays reach in the southern 
hemisphere on June 21 and December 22? 

This change of face presented to the sun is manifested 
to us in the varying daily path of the sun through the 
heavens, as it swings back and forth, north and south, 
once a year. 

Realistic Exercise. — Admit a beam of direct sunlight through a 
hole in a curtain or shutter. Hold a book so that the beam strikes 
its surface perpendicularly ; incline the book and observe the lighted 
spot grow larger and less bright as the angle increases. From Fig. 9 
it is evident that on account of the curvature of the earth's surface the 
parallel rays of the sun must always strike perpendicularly at some spot 
and at all others more or less slantingly, and that the area covered by 
any given bundle of rays increases as their slant increases. Therefore 
as the daily path of the sun approaches the zenith its heating power in- 
creases, and as it recedes from the zenith decreases. 

The Inequality of Day and Night. — Consult the almanac 
and find the length of the longest day and the shortest day 
in your latitude. If you can get an English almanac, find 
the same thing for London. 

Figure 6 shows how the portion of the sun's path above 
the horizon varies with the seasons, and Fig. 10 shows why 
this is so. About December 22 much less than half the 
northern hemisphere is in the sunlight ; hence the parallel 
of latitude or path of rotation of any given place, as New 
York or London, is more than 'half in darkness, and the 
time required to pass through the night is longer than that 



THE EARTH IN SPACE 23 

required to pass through the day. In the southern hemi- 
sphere the reverse is true, and on June 21 the condition of 
each hemisphere is the reverse of that on December 22. 
At the equator the days and nights are always equal, but 
the inequality increases toward the poles. Thus the sun's 
long brush paints the earth with bands of heat which swing 
back and forth, following the march of the apparent path 
of the sun and bringing the change of seasons. 

Equinoxes and Solstices. — The elates on which the sun is vertical 
over the equator and the days and nights are of equal length, March 21 
and September 23, are called the Vernal and the Autumnal Equinox. 
The dates when the sun is vertical over the tropics, June 21 and De- 
cember 22, are called the Summer and the Winter Solstice. The earth 
is nearest the sun on January 2 and farthest from it on July 3. It moves 
faster in that part of its orbit which is nearest the sun, hence the north- 
ern summer, from March 21 to September 23, is about six days longer 
than the northern winter. 

Location upon a Sphere. — If we take a perfectly plain 
ball of any kind and mark a point upon it, we shall find it 
impossible to describe in any way the position of the point. 
If we spin the ball like a top upon the table or when sus- 
pended from a cord, and thus establish an axis, two poles, 
and an equator, the position of any point may then be 
determined and described by its distance from them. 

Latitude. — The angular distance north or south from 
the equator is measured and expressed in degrees of lati- 
tude from o° to 90 . If the earth were a perfect sphere, 
every degree of latitude would be g^ of the circumference 
of the earth ; but the polar diameter being twenty-six miles 
shorter than the equatorial diameter, the convexity of the 
earth grows gradually less from the equator to the poles, 
and a degree of latitude becomes -^^ of the circumfer- 
ence of a larger and larger circle. Latitude is the angle 
between the radius of curvature of the earth's surface at 



24 



THE PLANET EARTH 



any point and the plane of the equator. The degrees 
increase in length toward the poles (Fig. n), according 
to the table on p. 25. 
NO -„™ LE . Realistic Exercise. — ■ At the equator the 

Polestar appears just at the horizon, midway 
between the equator and the pole it is 45 above 
the horizon, and at the pole it is in the zenith. 
The altitude or angular distance of the Pole- 
star above the horizon at any place is equal to 
the latitude of that place. 

On March 21 and September 23 the sun is 
directly over the equator. Observe the noon 
altitude of the sun on either of these days and 
subtract it from 90 ; the remainder is the lati- 
tude of the place. 

Longitude. — It is evident that all places equally distant 
from the equator are situated upon a line parallel with the 
equator and have the same latitude ; and it is necessary to 




Degrees of lati- 
tude 




Fig. 12. — Parallels and meridians, 



determine the position of each place upon its parallel of 
latitude. ' This is done by drawing lines called metidians 
from pole to pole at right angles to the parallels, and meas- 
uring the angular distance of the meridian passing through 



STRUCTURE OF THE EARTH 2J 

The Science of Geography deals especially with the region 
of contact and interpenetration between the rock sphere, 
water sphere, and atmosphere, the home of plants, animals, 
and men. Its business is to describe and explain the distri- 
bution of all the features found there. While the special 
field of the geographer lies on what is commonly called the 
earth's surface, he avails himself of whatever is known con- 
cerning any part of the earth to assist him in explaining 
the distribution of surface features. 

The Centrosphere. — We have no direct knowledge of the 
interior of the earth. The deepest boring yet made (at 
Schladebach, Germany) is only about six thousand feet 
deep, and the deepest natural cut (the Grand Canyon of 
the Colorado) is about the same depth, which is hardly 
proportional to a pin scratch through the varnish of a 
globe. Yet certain inferences may be drawn with great 
probability concerning the condition of the centrosphere. 

The pressure within the centrosphere is very great. 
Every mass of matter in it sustains the weight of the col- 
umn of rock above it, as the foundation of the Washing- 
ton monument is under the pressure of the stones placed 
upon it. It is calculated that at the depth of one hundred 
and fifty miles the pressure is one million pounds per square 
inch, and at the earth's center thirty million pounds. 
Below a depth of eight miles the pressure is sufficient to 
crush the strongest materials ; therefore, no open space or 
cavity can exist there. 

The density of the centrosphere is much greater than that of the 
crust. The average density of the whole earth has been determined by 
various methods, and it is thus shown that the earth weighs 5.6 times 
as much as an equal globe of water. But the average density of the 
rocks forming the crust is only between 2.5 and 3. Therefore, the 
material composing the centrosphere must be, on the average, about 
twice as dense as the crust. The supposition that this material may 



28 THE PLANET EARTH 

be composed largely of iron, gold, and other heavy metals, is not 
unreasonable. 

The temperature of the centrosphere is very high. In 
wells, mines, and tunnels the temperature is always found 
to increase downward. The rate of increase is variable, but 
averages i° for every fifty or sixty feet. It is probable that 
at great depths the temperature does not increase so rap- 
idly, and that below one hundred and fifty miles the centro- 
sphere has a nearly uniform temperature of about 7000 F. 

The centrosphere may be liquid, or solid, or partly liquid and partly 
solid. If the temperature increases downward at the rate of i° for every 
fifty or sixty feet, at a depth of fifty miles it is hot enough to melt any 
substance known upon the surface of the earth. The eruption of melted 
rock from volcanoes in many parts of the world has encouraged the belief 
that inside of a thin solid crust the earth is a liquid mass. But people 
who have held this opinion have failed to take into account the fact that 
pressure raises the melting point of most substances, and at some depth 
not very great the pressure may be sufficient to prevent the rock from 
melting, in spite of its high temperature. 

The attraction of the sun and moon pulls the sea out of shape and 
produces a regular rise and fall of the water known as the tides. If the 
centrosphere were liquid, it would be drawn out of shape in the same 
way, and the crust would heave up and down as the surface of the sea 
does. There is no evidence of such a movement ; on the contrary, 
Professor Darwin and Lord Kelvin have shown that the earth keeps its 
shape as rigidly as though it were a globe of solid steel. This would 
not be possible for an earth built on the plan of an egg — a thin shell 
filled with liquid. The jar of an earthquake in Japan is often felt in 
England, and the time which it occupies in passing through the earth 
indicates that it passes through a solid and not a liquid. 

The crust of the earth is not stretched smoothly over it like the skin 
of an apple, but is very much wrinkled, folded, and crumpled, especially 
in mountainous regions. There is some difficulty in conceiving how the 
crust could become crumpled over a solid globe ; but experiments have 
shown that under great pressure the most rigid solids, such as steel, act 
as if plastic and yield slowly to pressure like butter. Consequently the 
centrosphere, although solid, may. act as if plastic, yielding enough to 
permit all the movements which have taken place in the crust. 



STRUCTURE OF THE EARTH 29 

Volcanic eruptions may be accounted for by supposing that the lava 
comes from cisterns of liquid rock of no great extent, or that the rock 
melts in certain places where the wrinkling of the crust partly relieves 
it from pressure. Some geologists have been led to suppose that 
between the solid crust and the solid core there is a shallow layer of 
liquid matter all around. 

The condition of the centrosphere is necessarily 
shrouded in much uncertainty ; but the theory which 
seems to account best for all the facts supposes the 
existence of 

(1) A very large, dense, and hot core, solid because of 
the pressure upon it. 

(2) A surrounding shallow layer of liquid or semi- 
liquid matter, upon which the solid crust or rock sphere 
floats. 

So far as the structure of the earth is concerned, it may 
be roughly compared to a hot iron ball coated with tar 
and covered with wrinkled leather. Sir William Dawson 
compares the earth to a plum, peach, or cherry somewhat 
dried up ; it has a large, hard stone or kernel, a thin pulp, 
and on the outside a thin skin ; it has shrunk slightly, so 
as to produce wrinkles in the skin, and in some places the 
skin has cracked, allowing small quantities of the pulp to 
ooze out. 

The Earth-crust ; Mantle Rock. — It is a matter of com- 
mon observation that the crust of the earth is made up 
of a variety of materials. This may be seen almost any- 
where : in a plowed field, in an excavation for a cellar or 
ditch, in a cut made for a wagon road or railroad, in the 
banks of a stream, in a gravel pit or quarry. In lowlands 
and valleys, and on gentle slopes, the surface materials 
lie in a loose, incoherent mass, which may be removed 
with spade and pick. This loose and workable material 
is often called soil ; but it is better to use that term for 



30 THE PLANET EARTH 

only the upper layer of it, and to call the whole mass, 
however deep, mantle ivck, because it overlies and covers 
the other rock. The common varieties of mantle rock are 
clay, sand, gravel, pebbles, and boulders. They are all 
fragments of older rocks which have been broken up, and 
wholly or partly pulverized and decomposed. 

Clay is a soft, almost impalpable powder like flour, sticky and greasy 
when wet, and easily molded into any desired form. 

Sand is a mass of loose, hard grains, usually of the mineral quartz. 
The grains may be coarse or fine, sharp or rounded, but sand is always 
recognizable by its harsh, gritty feel. 

Gravel is composed of small stones, larger than sand grains. The 
stones may be angular or rounded, and of any color or composition, 
but are usually hard and smooth. 

Pebbles and Boulders are large fragments of any kind of rock which 
have been broken off and moved from the parent bed. 

Mixtures of clay, sand, gravel, and pebbles occur in all proportions. 
A mixture of sand and clay is commonly called loam ; when it contains 
also a portion of decayed vegetable matter (humus) it forms a fertile 
soil. 

Marl is a whitish mud found at the bottom of some lakes and ponds. 
It is a mixture of clay and lime derived largely from the decay of the 
shells of mussels, snails, and other animals. 

Peat or muck is a black mud formed by the decay of vegetable 
matter under water. It sometimes accumulates in lakes and marshes 
to the depth of many feet. 

Realistic Exercises. — Collect as many kinds of mantle rock as pos- 
sible and examine them carefully. Note the feel, color, odor, and taste 
of clay, and its behavior when wet. Under a good microscope the 
powder of dry clay is seen to consist of minute, flat, translucent scales. 
Examine different specimens of sand with a hand magnifier, and note 
the size, shape, and color of the grains. Shake up loam or a mixture 
of sand and clay in a tall bottle of water and let it settle. The sand 
will go to the bottom very quickly, while the clay will settle slowly 
on top of the sand : it makes the water turbid or roily, and it may 
take twenty-four hours for all the clay to settle and leave the water 
clear. 

Examine specimens of clean gravel : it may be necessary to wash 



STRUCTURE OF THE EARTH 



31 



out the sand and clay in a stream of water. From a pint or quart of 
gravel pick out the smooth, rounded stones and count them ; count the 
rough, angular stones, if any, and determine what per cent they are of 
the whole number. Try the hardness of each stone with the point of 
a knife and determine what per cent of them are soft, that is, easily 
scratched with a knife, and what per cent are hard, that is, scratched 
with difficulty or not at all. Bear in mind for future investigation the 
question, Why are most gravel stones hard and smooth ? 




Fig. 14. — Shale overlain by mantle rock. 

(Near Terre Haute, lnd.) 

Examine as many boulders as possible : note their size, shape, and 
hardness, and whether they are composed of only one kind of mineral or 
of several kinds. The shape may be rounded, without flat faces ; sub- 
angular, with flat faces and rounded edges and corners ; or angular, 
with sharp edges and corners. 

Examine a specimen of marl and note the fragments of shell. In 
peat observe the fragments of roots, stems, and leaves. 

Bed Rock. — If we dig or bore down far enough into the 
mantle rock, it will be found to be of moderate depth, and 



32 



THE PLANET EARTH 



to be everywhere underlain by bed rock. This is a solid, 
massive, continuous sheet of rock which extends indefinitely 
in every direction and requires the drill and hammer, or 
even the use of gunpowder or dynamite, for its removal. 
On mountains and in regions of steep slope the bed rock 

is very thinly covered or 
lies bare and exposed to 
the weather. A projection 
of bed rock through the 
mantle rock often occurs 
upon a hillside or cliff, 
and is called an outcrop 
or exposure. Bed rock is 
also likely to be exposed 
in the bed or along the 
banks of a stream. Over 
the greater part of the 
land surface bed rock is 
found to lie in distinct 
layers. A set of layers 
of one kind of rock is 
called a stratum (plural, 
strata). Shale, sandstone, 
conglomerate, and lime- 
stone are the only common kinds of stratified bed rock. 

Shale or mud rock (Fig. 14) is nothing but compacted and hardened 
clay. It is soft and fine-grained, Has the feel, odor, and taste of clay, 
splits easily into thin, irregular blocks, and is popularly, although 
wrongly, called " soapstone." It is usually of a gray or brown color, 
but may contain vegetable matter enough to make it black. The harder 
and more compact varieties of shale split into thin, regular leaves and 
are commonly called slate. 

Sandstone (Fig. 15) is composed of sand grains held together by 
some kind of cement. It may be fine- or coarse-grained ; tough and 




Fig. 15. —A sandstone cliff. 

(Montgomery County, Ind.) 



STRUCTURE OF THE EARTH 



33 




Fig 16. —Fragment of conglomerate. 



compact or loose and friable, so that it can be crumbled with the 
fingers. The colors vary from nearly white through gray and brown to 
dark red. The granular structure may be easily recognized by the 
harsh feel or the appearance under a magnifier. The most common 
cements in sandstone are clay, lime, 
silica, and iron, to which the red color 
is due. Grindstones and whetstones 
are made of fine and even-grained 
sandstone in which the cement is silica. 
The layers may be many feet in thick- 
ness or as thin as pasteboard. They 
often contain glistening grains of mica. 
Conglomerate is cemented gravel 
and is often found in the lower part of a gravel bed. If the pebbles are 
rounded, it is called pudding stone ; if angular, breccia. 

Limestone (Figs. 17, 18), the most abundant of all rocks, is largely 
composed of the skeletons or shells of animals which lived in water. 
These may be microscopic, as in chalk, or easily recognizable as shells, 
corals, crinoid stems ("button molds"), or other forms. Limestone is 
of all colors and is not too hard to be scratched 
with a knife. A drop of cold dilute hydro- 
chloric acid placed upon limestone dissolves it 
with foaming, due to the escape of gas. Some 
limestones are formed by the deposit of lime 
from solution in water, as the stalactites and 
stalagmites in caves, and the tufa around the 
mouths of some springs. Many limestones 
are composed of small rounded grains in ap- 
pearance like fish eggs. This kind is called 
concretionary or oolitic limestone. 
Bituminous coal, or soft coal, is a stratified bed rock formed by the 
consolidation of peat. 

Aqueous or Sedimentary Rocks. — All the rocks thus 
far described, including both mantle and bed rocks, are 
composed of materials which, in most cases, have been re- 
moved from their original position and carried some dis- 
tance by running, water. When the current of a stream 
slackens its speed or enters the still water of a lake or the 

DR. PHYS. GEOG. — 3 




Shell. 



Tufa. 



Fig. 17. —Specimens of 
limestone. 



34 



THE PLANET EARTH 



ocean, the mud, sand, and gravel which the stream has 
been carrying settle to the bottom. By this process the 
materials are deposited in nearly level layers or strata and 
are generally assorted so that each layer is composed chiefly 
of one kind of sediment. Hence all rocks thus formed 

are classed together as 
aqueous, sedimentary, or 
stratified rocks. 

Igneous and Metamor- 
phic Rocks. — If at any 
place we bore down 
through the mantle rock 
and through the layers 
of stratified bed rock, at 
a greater or less depth 
we strike bed rock which 
is not stratified and 
which owes its form and 
structure to the action 
of heat. In volcanic re- 
gions, melted rock has 
escaped in vast quanti- 
ties through cracks in 
the crust, spread out 
over the stratified rocks, 
and cooled in the form of 
lava streams and sheets. 
It is often blown out of 
volcanoes in the form of lava dust, sand, or gravel. Such 
rock is called eruptive or volcanic. In many cases the 
melted rock has not succeeded in reaching the surface, 
but has forced itself into the cracks and between the lay- 
ers of stratified rock and solidified there at considerable 




Fig. 18. — Limestone cliff. 

(Near Madison, Ind.) 



STRUCTURE OF THE EARTH 



35 



depths. Such rock is called intrusive. All rocks which 
have cooled from a once molten condition are called igneous. 
Other rocks have not been melted, but have been more 
or less changed from their original fragmental and strati- 
fied form by heat and pressure, and hence are called meta- 
morphic or altered rocks. Igneous and metamorphic rocks 
are distinguished from aqueous rocks by being often un- 
stratified and made up, not of fragments, but mainly or 
wholly of crystals. The crystals may be too small to be 
seen by the naked eye, but they are often conspicuous from 
their shape and sparkling luster. Some igneous rocks are 
not granular or crystalline, but structureless like glass. 

Granite is a common representative of a large family of igneous rocks, 
each member of which is composed of a mixture of two or more differ- 
ent minerals, rather coarsely crystallized and presenting a speckled or 
mottled appearance. Granite commonly consists of three minerals : 
( i ) quartz, in glassy, lustrous grains too hard to be scratched with a 
knife ; (2) feldspar, in white or reddish crystals 
which break with flat faces and square angles ; 
(3) mica, in soft, thin flakes, usually black. 
The mica is sometimes replaced by hornblende, 
in greenish or black prismatic or needle-shaped 
crystals. A mixture 
of feldspar with horn- 
blende or mica, but 
without quartz, is called 
syenite. If the minerals 
named above as form- 
ing granite are not 
thoroughly mixed, but 
occur in more or less 
regular bands, the rock is called gneiss. A mixture containing a large 
proportion of mica in coarse flakes, and therefore splitting easily, is 
called mica schist. 

Basalt or trap rock is a common representative of a group of minutely 
crystalline or glassy igneous rocks which are usually of a greenish or 
black color. 





Gneiss. 



Granite. 



Fig. 19. — Igneous rocks. 



36 



THE PLANET EARTH 



Realistic Exercise. — Collect as many specimens of rock as possi- 
ble, both stratified and unstratified. Almost anywhere north of the 
Ohio and Missouri rivers a gravel pit will furnish a hundred different 
kinds. Try first to separate the aqueous rocks from the igneous and 
metamorphic The latter are the more difficult, and to make much 
progress in studying them a descriptive handbook with a few labeled 
specimens are necessary. 1 

Classification of Common and Typical Rocks 



Origin 


Class 


Texture 


Bed Rock 
Consolidated 


Mantle Rock 
Unconsoli- 
dated 


Aqueous Rocks 
Deposited by water 


Mechanical 
Sediments. 


Fragmental. 


Shale. 

Sandstone. 

Conglomerate. 


Clay. 
Sand. 
Gravel. 


or ice. Usually 
stratified. 


Chemical or 
Organic 
Sediments. 


Crystalline, 
Compact. 


Limestone. 
Bituminous 
Coal. 


Marl. 
Peat. 


Igneous Rocks 
Cooled from a 


Eruptive or 
Volcanic. 

Cooled on the 
surface. 


Compact or 

Crystalline. 
Glassy. 


Basalt. 

Trap (Lava). 
Obsidian, Pumice. 


melted state. 
Unstratified. 


Intrusive. 

Cooled 
below the 
surface. 


Crystalline. 


Granite. 

Syenite. 




Stratified. 


Slaty. 
Compact. 
Crystalline. 
Glassy. 


Rock Name 


Original Form 


Metamorphic Rocks 
Altered by heat and 


Slate. 
Quartzite. 
Marble. 
Anthracite 
Coal. 


Shale. 
Sandstone. 
Limestone. 
Bituminous 
Coal. 


pressure. 


Unstratified. 


Banded. 
Schistose. 


Gneiss. 
Mica Schist. 


Conglomerate 
or Granite. 

Shale or 
Granite. 



i The Washington School Collection, furnished by E. E. Howell, Washington, 
D.C., is very good. 



STRUCTURE OF THE EARTH 37 

The Structure of the Earth-crust. — The crust of the 
earth, so far as, accessible to us, consists of three general 
layers. 

(1) On the outside, mantle rock, a layer of loose, uncon- 
solidated, generally stratified fragments, nowhere more 
than a few hundred feet thick, and in many places entirely 
wanting. 

(2) A layer of stratified and consolidated rock frag- 
ments, perhaps averaging five or ten miles in thickness. 
In mountainous regions it has been extensively warped, 
crumpled, and broken, and in some localities removed by 
erosion. 

(3) A fundamental, unstratified layer of unknown 
thickness, which has cooled and crystallized from a pre- 
viously molten condition. It has also extensively pene- 
trated into and through the other layers, and has thus 
become surface rock in some places. 



CHAPTER III 

THE FACE OF THE EARTH 

" If an observer from the depths of celestial space could observe the 
surface of our globe as it would present itself to him in the course of a 
daily rotation, the most striking feature would be the gradual narrowing 
of the continents toward the south." — Suess. 

The large and striking features of the face of the earth 
are due to the fact that the surface of the earth-crust is 
slightly irregular, and the waters of the sea have accumu- 
lated in its depressions. Those portions of the crust which 
project above the water level form the land masses, great 
and small, continents and islands. The shore line of the 
sea is the boundary between two strongly contrasted 
regions : the land with its varied surface and products, 
and the unbroken expanse of the ocean. 

Seventy-two per cent of the surface of the solid earth 
seems to be hidden from observation under a barren 
blanket of water. But apparatus has been invented by 
which we may feel down through the water and gain con- 
siderable knowledge of the solid crust beneath ; so that we 
can now represent to our imagination the main features of 
the earth as they would appear without water in the sea. 
On the relief map of the world, pp. 40, 41, the most promi- 
nent feature is the crookedness of the lines which mark the 
different levels and the consequent irregularity of the areas 
which they inclose. The largest tracts of approximately 
uniform level occur on the sea bottom between six thou- 
sand and eighteen thousand feet below the surface (medium 
shades of blue). Figure 20 shows the proportions of the 

38 



THE FACE OF THE EARTH 



39 



earth-crust which lie at the various levels. More than half 
(57 per cent) of its surface lies under water six thousand 
feet or more in depth. This is called the area of depres- 
sion or deep sea floor. The dry land (red and white) rests 
upon a block or foundation (lightest blue) a little larger 
than itself, which rise^-rather steeply from the sea floor. 
This continental block, comprising the land and the narrow 
belt of sea bottom around it, over which the water is less 
than six thousand feet deep, is called the area of elevation. 




Per Cent of Ar.ea of Earth-Crust Surface (10$ =19,700,000 Sq. M.) 
Fig. 20. — Generalized profile of the earth-crust. 
(Hypsographic curve — after Wagner.) 

Realistic Exercise. — Upon a hollow rubber ball five or six inches in 
diameter mark the poles and equator. Draw meridians and parallels 30 
apart : using these as guides, mark the outline of the continental block, 
including within it the Gulf of Mexico, Caribbean Sea, Mediterranean 
Sea, Red Sea, the seas between Australia and Asia, and the Arctic 
Ocean. Cut the ball along the outline marked : one portion thus made 
will have the form of the area of elevation (the Arctic Ocean being at 
the center), and the other portion the form of the area of depression. 

The Plan of the Earth. — The continental block sur- 
rounds the Arctic depression at a distance of about 20 



160 180 160 140 120 100 




4 



THE PLANET EARTH 



from the north pole, and extends thence south- 
ward in three great arms. The longest arm is 
occupied by North and South America, a land 
mass which extends to 56 south latitude. The 
second arm is occupied by Europe-Africa, which 
ends at 35 south, and the third arm by Asia- 
Australia, which ends at about 40 south. The 
south polar regions are probably occupied by land, 
which projects in a few places a little beyond the 
Antarctic Circle, but north of this land the sea 
forms a belt around the globe, from about io° to 
30 in width, and sends northward three great 
arms which interlock with the arms of the land. 
The longest arm, the Atlantic Ocean, lies between 
the Americas and Europe-Africa, and is wide open 
at both extremities, forming a channel of com- 
munication between the two polar regions. The 
broadest arm, the Pacific Ocean, lies between the 
Americas and Asia-Australia. It is nearly closed 
at latitude 65 north. The third arm, the Indian 
Ocean, lies between Africa and Australia, and ex- 
tends only to about 25 north latitude. 

In this plan there are several striking peculiarities. 
(1) There is a large excess of water in the southern hemi- 
sphere. (2) Each of the three continental arms is more or 
less broken by deep inlets of the sea which serve to separate 
them naturally into grand divisions of the land. The Ameri- 
can arm is thus broken at io° north, the Europe-African 
at 35 north, and the Asia-Australian near the equator. 

(3) Most of the resulting land masses are triangular, with 
bases to the north, and tapering points toward the south. 

(4) The great land masses and ocean basins are set over 
against each other on opposite sides of the globe. Europe- 
Africa is opposite the Pacific, Asia-Australia opposite the 
Atlantic, and North America opposite the Indian, while South 



THE FACE OF THE EARTH 43 

America forms an exception in being opposite to the island region of the 
western Pacific. This antipodal arrangement of land and water has led 
some geographers to think that the earth is a spheroid slightly flattened 
on four sides, which determine the positions of the Arctic. Atlantic, 
Pacific, and Indian depressions, while the land masses, including the 
Antarctic continent, occupy the projecting edges and corners. 

The Region of Depression. — More than three fourths of 
the sea floor (78 per cent) lies between 6000 and 18,000 
feet below sea level. Its generally smooth surface is 
broken by a few narrow ridges which support groups or 
chains of islands, and by valleys or holes (colored darkest 
blue on the map), called deeps, a few of which exceed 
24,000 feet in depth. The average depth of the sea is 
about 11,500 feet, or nearly 2\ miles. The greatest depth 
yet found is about 31,600 feet, near the Ladrone Islands 
in the western Pacific. 

The Region of Elevation. — From the deep sea floor the 
continental block rises in most places rather steeply, so 
that its sides, comprising the region between 600 and 6000 
feet below the sea level, form only 10 per cent of the area 
under water. Its upper surface is but slightly elevated, 
nine tenths of the land being less than 6000 feet above 
sea level. 

Much ingenuity has been expended in the effort to discover some 
unity of plan in the relief of the several grand divisions, but without 
much success. Continents do not take the form of raised domes plun- 
ging steeply on all sides toward the sea ; neither is there a central back- 
bone of high land, a feature very prominent in large islands ; nor are 
nnrginal highlands with a central depression between them the com- 
mon rule. South America is bordered on the western side by the 
narrow wall of the Andes Mountains. In North America the two par- 
allel systems of the Rocky Mountains and the Sierra Nevada-Cascade 
form a double barrier. The southeastern and southern parts of Asia 
are occupied by an irregular patchwork of lofty plateaus and mountains, 
which are prolonged through southern Europe to the Atlantic. In 
Africa and Australia the principal highlands lie along their eastern 



44 



THE PLANET EARTH 



sides. The only general plan of continental relief consists in the occur- 
rence of an elevated margin next to the Pacific and Indian oceans, and 
of extensive lowlands next to the Atlantic and Arctic oceans. 

The Coast Shelf. — The gentle slope of the lowlands often extends 
out to sea, forming a wide submerged shelf between the shore and the 
boundary of the steep slope of the continental block. Along a great part 
of the Atlantic coast the thin edge of the sea thus transgresses upon 
the land. On the other hand, the steep slope of the highlands near the 
Pacific coast usually continues under water, and the sides of the conti- 
nental block plunge abruptly downward to the deep sea floor. The un- 
broken descent from the summit of the Andes to the sea floor amounts 
in some places to a fall of 42,000 feet in 80 miles. The slopes of the 
Gulf of Guinea are in some places as much as 2000 feet per mile. 



50 Miles 100 




Fig. 22. — Profiles of coast shelves. 



Irregularity of Land Surface. — The surface of the land 
is characterized by general roughness and irregularity, in 
contrast with the comparative smoothness of the sea floor. 
A profile of the land almost anywhere, drawn upon a scale 
large enough to show the smaller features, appears some- 
what jagged, and in elevated regions it may resemble the 
teeth of a saw. 

An examination of continental relief in detail reveals the 
presence of numerous inclosed basins nearly or wholly 
surrounded by mountains, especially in the central regions 
of Asia-Europe and Africa, and in western North Amer- 
ica. As much as one fifth of the whole land surface is 
so inclosed as to have no outlet for drainage to the sea. 



THE FACE OF THE EARTH 45 

A few of these depressions in the land lie below sea level ; 
of these the basin of the Caspian Sea is the largest and 
that of the Dead Sea the lowest. 

More than one fourth (26.7 per cent) of the land surface 
of the globe lies between sea level and 600 feet, and 
nearly three fourths (73.7 per cent) below 3000 feet. The 
greatest height yet measured is Mount Everest in the 
Himalayas, 29,000 feet. The average elevation of the land 
is approximately 2300 feet, or a little less than half a mile. 

Characteristics of the Grand Divisions. — In Europe more 
than half the surface (54 per cent) is less than 600 feet 
above the sea and only one tenth more than 6000 feet. 
It is the least elevated of all the grand divisions and is 
characterized by extensive low plains. Of all' the grand 
divisions Africa has the smallest part (12.5 per cent) of 
its surface below 600 feet. It is characterized by extensive 
plateaus. Australia resembles Africa, but its elevation as 
well as its area is much less. Asia is distinguished by the 
height and massiveness of its mountain chains, which give 
it the greatest absolute height, 29,000 feet, and the greatest 
average height, 2884 feet. Thirty-eight per cent of it lies 
above 3000 feet. The Americas exhibit all the great 
relief forms, low plains, high plateaus, and mountain chains, 
without marked predominance of any. Their long north 
and south mountain systems are not continuous, but are 
separated in Central America by a system now partly 
submerged, but reappearing eastward through the West 
Indies. 

Islands. — All the large islands except Madagascar and 
New Zealand, and many small ones, stand upon the conti- 
nental block and seem to be the tops of peaks or ridges 
rising from its partly submerged surface. Oceanic islands 
rise from the region of depression and are of volcanic origin. 



46 THE PLANET EARTH 

Comparative Smoothness of the Crust Surface. — In comparison 
with the size of the earth the irregularities of the crust are trifling. The 
lowest point known is a little over 31,000 feet below sea level and the 
highest point is 29,000 feet above sea level, so that the range of relief 
or vertical distance between them is only about 60,000 feet, 11^ miles, 
or 7 ^g of the earth's diameter. Upon a globe 7 feet in diameter the 
range of relief proportional to that of the earth would be one eighth 
inch, which is considerably less in proportion than that of the roughness 
of the skin of an orange. If the elevations of the crust were used to fill 
the depressions and the whole surface were graded to one level, that 
surface would be 1 .44 miles below the present sea level and would be 
covered with water 1.56 miles deep. 

Sea and Land. — The sea itself, with an average depth of 
about 2\ miles, is only a thin skin upon the globe which, 
like a shallow pool upon a sidewalk after a rain, serves to 
mark the outlines of a depression which would be otherwise 
scarcely noticeable ; yet its volume is nearly thirteen 
times as great as that of the land above water. The posi- 
tion of the water surface or sea level determines the most 
important boundaries of the world. From it all heights 
and depths are measured, and by it all coast lines are fixed. 
A slight rise of sea level would submerge large areas of 
land and change entirely the outlines of continents ; while 
a lowering of six thousand feet in its level would not 
materially change the outline of the continents, but would 
unite them into a single mass. 

Causes of Relief. — The causes of irregularity in the sur- 
face of the earth-crust are not fully understood. This prob- 
lem has been the subject of much study and speculation, 
and many hypotheses have been proposed to account for 
the depression of the deep sea floor and the elevation of 
the continental block. 

Diastrophism. — The upper layers of the earth-crust on land are 
composed largely of sedimentary rocks, such as are now forming on the 
sea bottom near shore, and they contain the fossil remains of animals 



THE FACE OF THE EARTH 47 

which live only in shallow sea water. Hence we feel sure that nearly 
all the present land surface was once covered by the sea and has been 
raised from that position to its present elevation. The sedimentary 
strata have not only been raised bodily hundreds or thousands of feet, 
but they have, also been broken, tilted, folded, crumpled, and crushed 
together in a manner which shows that they have been subjected to 
enormous horizontal pressure (see pp. 178, 192). The movements of ele- 
vation, depression, fracture, and dislocation are still in progress. Marks 
placed upon the rocks along the coast of Sweden many years ago show 
that the land is slowly rising, in some places three or four feet in a cen- 
tury. Buildings erected three or four hundred years ago on the west 
coast of Greenland are now under the sea. Buried stumps of trees on 
the coast plain of New Jersey furnish evidence that a slow sinking is in 
progress there. A Spanish magazine built near the mouth of the Mis- 
sissippi about two hundred and thirty years ago is now more than ten 
feet under water. During an earthquake in 1822 the coast of Chile was 
suddenly raised two or three feet, and again an equal distance in 1835. 




Fig. 23. — Cape Maysi, Cuba. 

The coast of Cuba presents a series of raised benches which were cut' 
by the waves when the land stood at lower levels than now. Any solu- 
tion of the problem of the causes of diastrophism, or movement in the 
earth-crust, must account for the elevation of great continental areas 
and for the crumpling and breaking of the strata. 

Isostasy. — It is thought by many- geologists that the earth-crust 
under the ocean is denser and heavier than it is under the land, and 
that in consequence the sea floor sinks more deeply into the underlying 
centrosphere while the continental block is floated higher above it. 

Realistic Exercises. — Float two blocks of wood of the same size and 
shape, one of oak and the other of pine, in a vessel of water, and note the 
different heights of their upper surfaces above the surface of the water. 

Fill a U-shaped glass tube nearly half full of water. Pour oil into 
one arm and note the different levels at which the liquids stand. 
The shorter column of relatively heavy water balances the longer 
column of relatively light oil and holds its surface up to a higher level. 



48 THE PLANET EARTH 

Several lines of evidence seem to indicate that the material compos- 
ing the great plateaus and mountain ranges of the world is actually 
lighter than the average of the earth crust. These regions are probably 
not held up at their high level by the rigid support of the surrounding 
parts of the crust, as the roof of a building is supported by the walls, but 
they are pushed up by the heavier masses of the sea floor, the pressure 
of which is transmitted in every direction, as if through a liquid or 
plastic layer beneath the crust. This balancing of sea floor and con- 
tinental block has been called by Dutton isostasy. 

Contraction. — The wrinkling and folding of the earth-crust has long 
been accounted for in another way. It seems certain that the earth 
was once much hotter than it is now, and that it is constantly radiating 
heat into the cooler space around it as a hot stone or iron radiates heat 
in a cold day. If the globe has been cooling, it must also have been 
contracting or growing smaller. The heat of the sun keeps the crust 
at a nearly constant temperature, but the centrosphere has kept on 
cooling and contracting until the crust has become too big for it and 
is compelled to fold and wrinkle by its own weight. The wrinkled 
skin of a withered peach or a cold baked apple is an example of a simi- 
lar change. 

Realistic Exercise. — Inflate a rubber toy balloon with air, cover it with 
a thin layer of flour paste, and rotate it in flour until a smooth dry coat- 
ing is formed an eighth of an inch thick. Attach to it by a glass con- 
nector a rubber tube provided with a pinchcock. Immerse the end of 
the tube in water, and let the air escape from the balloon a few bubbles at 
a time. As the balloon contracts, the coating of paste will become 
folded and wrinkled in a manner quite similar to the folding of the 
earth-crust. 

The hypothesis of isostasy, or balancing weights, seems best to 
account for the great regional elevations and depressions (continents 
and ocean basins), and the hypothesis of cooling and contraction best 
to account for such smaller features as mountain ranges. 

The Representation of Relief. — The facts of geography 
are most conveniently expressed by the use of maps. The, 
fundamental idea of a map is a drawing which shows npon 
a horizontal plane the location, direction, distance, and area 
of the features of the earth's surface as they are distributed. 
A plan of a house showing the arrangement, shape, and 



THE FACE OF THE EARTH 49 

size of the rooms, doors, windows, etc., and perhaps the 
location of pieces of furniture, is an example of a simple 
map. More complex maps may be made not only to show 
arrangement "on the flat," but also to indicate the "ele- 
vation " or relief of the surface. The map on pp. 40, 41, 
makes use of a common device for showing relief. The 
areas of different elevations are printed in different colors, 
various shades of blue being used for the sea floor, and 
shades of red for the land. Each boundary line of a color 
or shade is level or everywhere at the same distance above 
or below the sea level, measured vertically. These lines 
of equal elevation upon a map are called contour Hues or 
simply contours. 

The lightest shade of red shows all the land between sea level and 
three thousand feet above, but does not show where the land is just one 
hundred or twenty-nine hundred feet. The uncolored area shows all the 
land above twelve thousand feet, but does not show just how much above 
that level any point is. The boundary lines between the different colors 
or shades indicate exactly the elevation of the places through which they 
pass. The line between the red and the blue is the coast line and 
is everywhere at sea level, the line between the two lightest shades of 
blue is everywhere exactly six thousand feet below sea level, and the line 
around the outside margin of the darkest red is everywhere exactly six 
thousand feet above sea level. 

By drawing contour lines at sufficiently small intervals relief may be 
indicated with any desired degree of precision. When contour lines 
are drawn at small intervals the spaces between them are frequently 
left uncolored. 

Figure 24 shows a sketch or picture of a landscape and Fig. 25 a 
contoured map of the same region. In the foreground is a portion of 
the sea, the shore line of which forms the basal or zero contour. Con- 
tours are drawn upon the map at intervals of fifty feet measured verti- 
cally from the sea level, and they mark the lines where the seashore 
would be if the sea should rise fifty, one hundred, etc., feet. Where 
the slope is steep, one would have to travel only a short distance to rise 
fifty feet ; hence the contours are close together. Where the slope is 
gentle, one would have to travel far to rise fifty feet ; hence the contours 



5o 



THE PLANET EARTH 



are farther apart. By shortening the contour interval to five or ten feet, 
as may be done upon a large-scale map, the elevation of every point 
may be shown very precisely. For showing exact elevation no device 
is equal to the contoured map ; but it has the disadvantage of not being 
graphic, that is, of not being understood by everybody at a glance. 




Fig. .24. 




Fig. 25. 

One must learn to interpret such a map before he is able to form a 
mental picture of the region shown. 

Realistic Exercise. — Upon a table or floor make a clay model of a 
simple hill with both steep and gentle slopes. Lay a block of wood one 
inch thick beside it : with a pointed stick make a mark upon the side 
of the clay hill all around at the height of the upper surface of the block, 



THE FACE OF THE EARTH 



51 



which should be moved around to guide the stick. Lay another block an 
inch thick upon the first and make another mark around the hill two inches 
above the table. Continue to add blocks until the top of the hill is 
reached. Each mark upon the hill will be one inch above or below the 
next, and will indicate exactly the height, above the table or floor, of 
the points through which it passes. Measure the horizontal distance 
between the marks upon the gentle and the steep slope. Now look 
down upon the hill from some distance above, and draw upon paper the 
lines as they appear from that point of view. The drawing will be a 
contoured map of the hill in which the contour interval is one inch. 





Fig. 26. 



Fig. 27. 



A very common device for showing relief upon a map is the use of 
hachures, or fine lines running up and down the slopes, and so drawn 
as to show the steepness of the slope by the depth of shading. Ha- 
chured maps may be made very graphic and almost equal to a picture. 
Figures 26 and 27 show the relation between hachured and contoured 
maps of the same area. A combination of the two is the best possible 
method of showing relief upon a map. 

Models are miniature reproductions of portions of the 
earth's surface in sand, clay, paper, plaster, or other mate- 
rial (see p. 393). They are often called relief maps. The 
vertical heights are usually exaggerated in order to show 
small details. This exaggeration is sometimes excessive, 
the elevations being made forty to one hundred times as 



52 



THE PLANET EARTH 




f^g 6" 'S 



high as they ought to 
be in proportion to the 
widths shown. Thus all 
slopes become so steep 
and the forms of moun- 
tains and other features 
so unnatural as to ren- 
der the model worse 
than useless because it 
teaches more error than 
truth. A good model, 
in which the heights are 
not exaggerated more 
than ten times, is gen- 
2 erally the best represen- 
2 tation of relief. Pictures 
1 of models are very good 
substitutes for the mod- 
E els themselves, but are 
subject to the same re- 
strictions in regard to 
exaggeration of heights. 
K profile shows the ele- 
vations and depressions 
of surface along any 
given line, which may 
be straight or crooked. 
It has two scales, the 
horizontal and the ver- 
tical. The latter is gen- 
erally exaggerated, and 
often greatly so. 

The student should 



THE FACE OF THE EARTH 53 

notice the amount of exaggeration in a model or a profile, 
and guard against the erroneous impression which it 
would otherwise give him. 

The stereogram, or block picture, is a combination of 
model and section, and may be used very effectively to 
show the relation of relief and structure. See Fig. 1 54. 

The Earth as the Home of Plants, Animals, and Men. — 
Life forms as we know them — plants, animals, and men 
■ — are able to live and flourish upon the earth because they 
have become adapted to a multitude of conditions which 
probably do not exist in the same combination upon any 
other planet. The most important conditions which make 
the earth habitable are dependent upon its position, form, 
attitude, motions, size, structure, and plan. 

The position of the earth — its distance from the sun — 
determines the amount of heat which it receives. This is 
sufficient to maintain at all places upon the face of the 
earth a temperature which never falls lower than about 
120 below the freezing point of water ( —88° F.), and never 
rises higher than about 120 above the freezing point 
(152 F.). This makes it possible for large quantities of 
water to exist in each of three forms, — solid ice, liquid 
water, and gaseous vapor. 

The form of the earth determines the angle at which the 
nearly parallel rays of the sun strike its face at different 
latitudes, and consequently the amount of heat received 
per square mile. This gives a variety of temperatures 
ranging from the torrid to the frigid. 

The attitude of the earth, or the inclination of its axis, in 
combination with its daily and yearly motions, determines 
a change of seasons, or variation of temperature, at all 
latitudes, and prevents both the uniformity which would 
exist if the earth's axis were perpendicular to the plane 

DR. PHYS. GEOG . — 4 



54 THE PLANET EARTH 

of its orbit, and the excessive variation which would result 
if the axis were nearly parallel to that plane. 

The revolution of the earth around the sun at a nearly- 
uniform speed in an orbit which is nearly circular brings 
about the regular succession of seasons and years, each of 
which is of moderate length. The succession of warm 
and cool, or wet and dry, seasons gives to plants and ani- 
mals alternating periods of comparative rest and activity. 

The rotation of the earth upon its axis exposes the greater 
part of its face to alternations of heat and cold, light and 
darkness, at short intervals, and imposes upon living beings 
correspondingly short and frequent periods of rest and 
activity. It also enables man to look out at night into 
space, see the moon and stars, and learn something of the 
universe of which the planet earth forms an insignificant 
part. 

The size and density of the earth determine its mass, 
or weight, and consequently the force of gravity. The 
attraction of the solid earth is sufficient to prevent the 
atmosphere from escaping into space and to give it such 
composition and density as to support plant and animal 
life. The attraction of the earth also determines the weight 
of every object upon its face, and the strength or rigidity 
of plants and the muscular power of animals are nicely 
adapted to support or to move their own and other weights. 

The structure of the earth gives a firm crust for the sup- 
port of all creatures which live upon the land, while the 
outer layer of pulverized mantle rock furnishes a permea- 
ble bed for the roots of plants and a storehouse of availa- 
ble food. Even the centrosphere contributes to the food 
supply ; for the masses of igneous rock which have 
escaped from it gradually crumble into mantle rock and 
are converted into fertile soil. The presence of large fluid 



THE FACE OF THE EARTH 55 

masses of water and of air makes it possible for extensive 
systems of currents to circulate in the atmosphere, in the 
sea, and on the surface of the land. Thus the materials of 
the earth are kept in motion and its face is made to un- 
dergo perpetual change. It is this which keeps the earth 
a living planet as distinguished from a dead one like the 
moon, and contributes to that variety and beauty of sky 
and landscape which make it a pleasant home for man. 

The sea is the home of millions of living forms, and it 
furnishes water for all those which live upon the land. 
The atmosphere not only rests upon the land, but pene- 
trates to the bottom of the sea and supplies all creatures 
with the breath of life. From it plants obtain the carbon 
which forms the greater part of their own substance, the 
food of animals, and the material of fuel. It absorbs and 
retains the heat of the sun, tempering its intensity by day 
and preventing its too rapid escape by night. Currents of 
air carry the water-vapor from the sea and distribute it 
as rain and snow over the land. The air and the water 
which falls from it attack the solid crust of the earth and 
break it up into mantle rock. 

The plan of the earth presents a vast expanse of water 
broken at intervals by large and small masses of land. 
While the land masses predominate in the northern hemi- 
sphere, their longer axes extend north and south through 
so many degrees of latitude as to traverse all the zones of 
climate. This variety is made still greater by diversities 
of elevation, relief, and distance from the sea. The num- 
ber and variety of living forms probably decrease from 
near sea level downward to the deep sea floor and upward 
to the mountain tops, but the great expanse of sea surface 
and the low average elevation of the land make a very 
large proportion of the face of the earth available for a 



56 THE PLANET EARTH 

dense population of some kind. The arrangement and 
relief of the land masses are such that the moisture evapo- 
rated from the sea is very unevenly distributed over them. 
Some portions receive an excess of rainfall, while exten- 
sive areas in every continent are so dry as to be very un- 
favorable to the existence of life. 

Land plants and animals are generally unable to cross 
oceans, deserts, or mountains, and the presence of these 
natural barriers has largely controlled the migration and 
distribution of man himself. If, from the whole face of 
the earth, those portions are deducted which are either too 
high, too low, too hot, too cold, too wet, or too dry, there 
remains not more than one tenth part which is suitable for 
the home of a dense population of civilized men. The 
infinite variety of situation, relief, soil, and climate has 
brought about a corresponding variety of living forms, each 
adapted to the peculiar set of conditions under which it 
lives. Probably no large part of the sea or land is entirely 
devoid of life ; but the sphere of life is strictly confined to 
the thin shell of the earth where land, water, and air inter- 
mingle. Not far below it lies the fervent heat of the cen- 
trosphere, and not far above it the intense cold of stellar 
space. 

Reference Books. — For a list of reference books on subjects included 
in Book I, see Appendix IV, especially pp. 413, 414. 



BOOK II. THE LAND 

The hills are shadows, and they flow 
From form to form, and nothing stands : 
They melt like mist, the solid lands, 
Like clouds they shape themselves and go. 

— lENNYSON'S In Memoriam. 

CHAPTER IV 
EROSION 

Weathering. — The contrast between the roughness of 
the land surface and the smoothness of the sea floor is due 
to the fact that the land is exposed to the action of the 
atmosphere, while the sea floor is protected from it. The 
direct action of the air and the weather upon the earth- 
crust is a complex process called weathering, accomplished 
and assisted by various agents. Its general result is to 
break up and crumble the surface of the land. 

(i) Oxygen is an active chemical agent which causes 
rocks to decay and crumble by a process like the rusting of 
iron. Carbon dioxide attacks igneous rocks and converts 
them into materials from which stratified rocks are formed. 

(2) Rainwater dissolves the cement present in many 
rocks, which in consequence fall to pieces. It also me- 
chanically removes and washes away loose particles. 

(3) Frost is one of the most efficient agents concerned 
in breaking up rock masses. When water freezes in the 
pores and crevices of rocks, it expands, and thus enlarges 
the cracks or makes new ones. When the morning sun, 
after a frosty night, strikes against a cliff, there is often a 

57 



58 



THE LAND 



continuous shower of rock fragments which have been 
loosened by the freezing and thawing. At high altitudes, 
where changes of temperature are frequent and severe, 
mountain peaks are rapidly broken down by this process. 

(4) Changes of temperature which do not include freezing 
and thawing act in a similar manner. When rock at any 
temperature is warmed it expands, and when cooled it con- 




Fig. 29. — Weathered granite. 
(Near St. Cloud, Minn.) 

tracts. Repeated expansion and contraction tends to break 
it up, especially to scale off thin sheets from the surface. 
This process is often used to break in pieces large boulders. 
They are first heated by a fire, and then suddenly cooled 
by throwing water upon them. 

(5) Gravity is continually pulling every mass of rock 
downward, and if the rock mass is insufficiently supported 



EROSION 



59 



it breaks off by its own weight. Gravity is not a part of 
the atmosphere, but in all processes of weathering it is a 
silent partner which never forgets or lets go for a moment. 

(6) Wind, by its own force, 
but more efficiently by blow- 
ing sand against rock, wears 
it away. Even the hardest 
materials, as steel and glass, 
are rapidly carved and eroded 
by wind-blown sand. 

(7) Plants and animals, 
although not a part of the 
atmosphere or factors of 
weather, may be included 
among the agents of rock 
disintegration. The roots of 
plants penetrate crevices in 
the rock and by their growth 
force the sides farther apart. 
Decaying vegetation fur- 
nishes an acid which in- 
creases the solvent power of 
water. Various burrowing (Near Prescott, Ariz.) 

and boring animals accomplish some less important work 
in rock destruction. 

By the action of all these agents large masses of rock are 
disintegrated and reduced to smaller and smaller fragments. 
Some rocks are also chemically decomposed and changed 
into other minerals. Weathering is the process by which 
massive bed rocks are converted into mantle rocks, and 
its products are clay, sand, gravel, pebbles, and boulders. 

Of these, only clay is a product of chemical decomposition and bears 
little resemblance to the original igneous or metamorphic rock from 




Fig. 3°. 



Cliffs eroded by wind and 
sand. 



60 THE LAND 

which it was produced. The other kinds of mantle rock are clearly 
recognizable as fragments of larger masses. Weathering is not con- 
fined to the surface of the earth-crust, but extends as far down as air 
and water penetrate. It is most active in the zone between the surface 
and the level of permanent ground-water. The bed rock is sometimes 
found to be broken up or " rotten " to the depth of one hundred feet or 
more. In some places the mantle rock lies undisturbed in the position 
where it was formed, and the transition from soil to bed rock is so 
gradual that it is impossible to tell where one ends and the other begins. 
(See Fig. 14.) More frequently the loose fragments have been re- 
moved some distance and deposited in another place. Mantle rock in 
place is called residual soil, while that which has been transported is 
often called drift, — wind, glacial, or stream drift, according to the 
agent of transportation. 

Weathering is most rapid and extensive (1) in regions of heavy 
rainfall, (2) at high altitudes, where changes of temperature, especially 
freezing and thawing, are frequent, (3) on steep slopes, where gravity 
acts most efficiently and the mantle rock promptly falls, slides, or is 
washed away, and (4) in regions of fragile or soluble rock. 

Realistic Exercises. — Examine pebbles and boulders and observe the 
difference between the weathered surface and the surface of a fresh frac- 
ture. One may be lighter or darker, rougher or smoother, than the other, 
according to the kind of rock. The depth to which weathering has pen- 
etrated is often plainly visible, and some specimens may be found in a 
crumbling condition throughout. If a cliff of bed rock is accessible, 
observe the talus or pile of fallen fragments which lies at its foot. Ex- 
amine the place of contact between the bed rock and the mantle rock 
above and observe whether the change is gradual or abrupt. If a section 
of bed rock is freshly exposed, as in a quarry, the depth to which air 
has penetrated is often shown by the staining of the rock along the 
cracks. 

Valleys and Streams. — It is hardly possible to travel 
anywhere upon the land surface without coming across a 
valley. Of all land forms it is the most common, — so 
common as to attract no attention unless it is unusually 
deep or otherwise troublesome to cross. Valleys exist in 
great variety and in all dimensions, from a barely visible 
furrow to a canyon a mile deep ; but almost without excep- 



EROSION 6l 

tion they are alike in having a stream at the bottom. This 
universal association of a stream with a valley does not 
excite surprise, because we expect water to flow along the 
lowest level. If valleys exist, it is but natural, as we say, 
that streams should find and follow them. But let us turn 
this proposition around and consider its reverse side. If 
streams exist, is it not natural that their courses should be 
marked by valleys ? 

Run-off. — If the course of a stream is followed up, it 
will be found to be joined at intervals on either side by 
tributaries, each of which flows in a valley usually propor- 
tioned to the size of the stream. The main stream and its 
valley grow smaller above the mouth of each tributary 
until they are reduced to a tiny rivulet flowing in a furrow, 
and finally come to an end at a spring, pond, or swamp, or 
upon the smooth slope of a hillside. If any tributary 
is followed up, it also is found to divide like the trunk 
of a tree into smaller branches and rivulets. The sur- 
face of the land on either side slopes toward the stream 
or one of its tributaries, and at the same time there is 
a continuous slope downstream from the head or tip of 
every branch. If the slope is ascended from the stream, 
at a greater or less distance a point is reached where the 
surface begins to slope away from that stream toward some 
other stream. A more or less definite line may be found 
which marks the junction of the two slopes and separates 
the water flowing into one stream from that flowing into 
the other. If this divide or water' parting is followed, it is 
found to pass around the heads of all the tributaries and 
to inclose the basin or area from which water drains into 
the stream system. Some part of the rain falling upon any 
basin sinks into the ground, but a large part runs down 
the slopes. At first this water forms a thin and scarcely 



62 



THE LAND 



perceptible sheet; but it soon gathers into little rills which 
join one another and grow larger until they flow into one 
of the permanent branches of the stream system. The 
smallest branches flow only while it rains, and their grooves 
or gullies are dry most of the time. The permanent branches 
are supplied from ponds, swamps, glaciers, or springs. 

Near the sources of the stream the slopes are apt to be steep, the 
current swift, the channel narrow and deep and perhaps interrupted by 

rapids and falls. The bed is 
strewn with boulders, pebbles, or 
coarse gravel. Farther down, as 
the slope becomes more gentle, 
the bed is smoother, rapids are 
less frequent, and are separated 
by long reaches of quiet water, 
and the channel becomes wider, 
shallower, and more crooked. 
The loose material is less coarse 
and consists chiefly of fine gravel 
and sand. Here the water course 
is likely to become double and 
to consist of a wide outer valley 
which the stream covers only at 
high water and through which 
the narrower channel winds irreg- 
ularly from side to side. Still 
farther down, the valley may be- 
come very much wider and con- 
sist of an extensive flood filaiti 
bounded by bluffs. Here the ordi- 
nary channel follows a meander- 
ing course, full of zigzag bends 
and horseshoe curves. The slope is gentle, the current sluggish, and 
the bed obstructed by sand bars and mud banks. The stream finally 
empties into a larger stream, or into a lake or the sea. 

These are the usual conditions of run-off or the escape 
of rainwater from a hydrograpJiic basin. 





ask ^^fedHWHcOfy 


















"Gfeft^ 




KpT*^ 


'-'""' '* iij 


Mr ■ ' A 


pdpsr? ■ 




wm^''''' : ' :[ ' i mm 




• ! '"- : - Jijfv' 


r jSk 



Fig 31— A mountain stream. 
(Rainbow Falls, Ute Pass, Colo.) 



EROSION 



63 




Fig. 32. — Small stream meandering in a flood plain ; sand bar and small terraces. 

Transportation. — But this is only half the story. It does 
not require a very close study of a stream to discover that 
it is not only a stream of water but also a stream of mantle 

rock, that the crust of 
the earth itself is flowing 
away through the same 
channel. Some streams 
are clear and some are 
muddy, but all carry a 
portion of mantle rock. 
The work of the stream 
is most rapid and most 
impressive at high water 
and in the upper parts 

Fig. 33. -Boulders in bed of stream. of itg CQms ^ where the 

current is swift it rolls and pushes pebbles along the bottom, 
and at times of flood is able to move even large boulders. 




6 4 



THE LAND 



In the middle course, where the current is moderate, it 
may be seen to carry sand or to roll it along the bottom, and 

the inside of every 
bend is marked by a 
deposit of sand left 
as the water went 
down after the last 
flood. In the lower 
course, where the 
current is very gentle, 
only clay or the finest 
sand is carried, all 
the coarser material 
having been dropped 
farther upstream. 




^0" 



Fig. 34. — Sand bar and bluff at bend of river. 
(Mississippi R., below St. Cloud, Minn.) 



Rock fragments of all sizes are buoyed up by the water so as to lose 
one third or more of their weight, and are in consequence more easily 
moved than when out of water. The current of a stream does not flow 
smoothly onward, but is disturbed by the irregularities of its bed so as 
to be thrown into ripples and eddies. This irregular motion helps to 
keep the fragments from settling. In a smooth, gently flowing river the 
mud may often be seen boiling up from the bottom in hundreds of 
places where an upward current comes to the surface. Even over a 
smooth bed the current has a wavy up and down movement which 
throws the sand at the bottom into cross ridges or ripples, with the 
longer slope upstream. The finer the particles of rock, the more slowly 
do they settle in water and the more easily are they kept in suspension ; 
therefore, sand is carried along by a slower and smoother stream than 
gravel, and clay by a slower stream than sand. The size and weight 
of the particles which a stream can carry in suspension increase rapidly 
with the velocity of its current. A current of one third of a mile an 
hour will carry clay ; of two thirds of a mile, fine sand ; of two miles, 
pebbles as large as cherries ; of four miles, stones as large as an egg. 

Material in suspension usually manifests itself by making the water 
turbid or muddy, but streams also carry invisible rock material in solu- 
tion. The most common materials transported in solution are salt and 



EROSION 65 

lime. It is dissolved lime which makes water hard, and leaves a crust 
on the bottom of a kettle in which hard water is boiled. The ability 
of a stream to carry material in solution is not affected by the velocity 
of the current, for the material is not deposited except by evaporation 
or some chemical change. 

Corrasion. — Wherever a stream runs over bed rock it 
wears the rock away slowly by solution, but a stream which 
carries sediment in suspension may wear away such rock 
rapidly. A clear stream acts upon rock like a piece of paper 
rubbed upon wood, a muddy stream like a piece of sand 
paper. If a stream is not overloaded with sediment and 
can carry sand or coarser particles rapidly, it cuts or files its 
way down into the crust of the earth through the hardest 
rocks. The grains of sand and gravel not only wear away 
the stream bed, but wear upon one another. Boulders and 
pebbles which are rolled and tumbled about in the current 
have their corners and edges worn off, and become smaller 
and more rounded as they travel on. Only the hard ones 
can. endure such harsh treatment without being reduced to 
powder. This explains why gravel stones are seldom 
angular or soft. 

Corrasion is most rapid where (1) the slope is steep, 
(2) the volume of water large, (3) the quantity of sedi- 
ment sufficient, but not too great, and (4) the bed rock 
soft or friable. 

Erosion. — All over the surface of a stream basin the 
crust of the earth is being crumbled to pieces by the 
agents of weathering. Gravity and the wash of the rain 
drag and push the mantle rock thus formed down the 
slopes into the stream. The current of the stream trans- 
ports the material to lower levels, and in doing so cuts 
its own channel deeper. Thus the land is everywhere 
being torn down and carried away toward the sea. The 



66 THE LAND 

lowering of the land surface by weathering, transportation, 
and corrasion is called erosion or degradation, and its most 
efficient agents are gravity and running water. In regions 
of small rainfall and steep slope, corrasion is more effect- 
ive than weathering, and erosion goes on much more 
rapidly near the streams, which cut their channels and 
valleys deeply into the face of the country. In regions of 
large rainfall and gentle slope, weathering and rainwash 
are more efficient than corrasion, and erosion is more uni- 
formly distributed over the basin, though still most rapid 
near the streams. 

Summary. — ■ From these facts it appears not only that 
a stream of running water is competent to make a valley, 
but that it must necessarily do so, and that a small stream 
is able to make a large valley if it is given time enough. 
The surface of the land is cut by innumerable valleys ; 
running water is the only agent everywhere present which 
is known to be capable of doing such work : therefore a 
stream valley is regarded as the depression or trench which 
the stream itself has cut. 1 Its bottom is or has been at 
some time covered by the stream, and it is bounded by 
relatively steep banks or bluffs. The channel sometimes 
occupies the whole width of the valley and sometimes 
only a small portion of it. 

Realistic Exercises. — Let the student visit any convenient stream 
and see for himself as many of the above described features and processes 
as can be found. Let him watch the stream in action, during or just after 
a rain, and see what it does with sediment under varying conditions of 
fineness and current. At some favorable point supply it with fine and 
coarse material and observe the result. Shake up clay, sand, and gravel 
in a tall bottle of water and observe the manner in which they settle. 
A stream a foot wide is doing the same kind of work in the same way 

1 Care should be taken to distinguish between the valley and the basin, which is 
popularly called valley. 



EROSION 6/ 

as the largest river, and one may be known by studying the other. It is 
of the greatest importance that stream action be actually studied in the 
field. 

"Every river appears to consist of a main trunk fed from 
a variety of branches, each running in a valley proportioned 
to its size, and all of them together forming a system of val- 
leys, communicating with one another, and having such a 
nice adjustment of their slopes that none of them join the 
principal valley either on too high or too low a level : a cir- 
cumstance which would be infinitely improbable if each of 
these valleys were not the work of the streams which filozv 
through them.''' — John Playfair, 1802. 



CHAPTER V 

THE MISSISSIPPI RIVER SYSTEM 



The basin drained by the Mississippi River and its tribu- 
taries has an area of about one and a quarter million square 
miles and is one of the largest in the world. On the west 




DRAINAG 

AND PRINCIFAL STREAMS 



Fig- 35 

it is separated from the basins of streams flowing into the 
Pacific by the crest of the Rocky Mountains. On the 
north the divide between it and the basins of the Nelson 
and the St. Lawrence is low and flat. On the east it is 

68 



THE MISSISSIPPI RIVER SYSTEM 



6 9 



3jnns5]; 



S[i[d 



A!0 




bounded by the Appalachian highland, and 
on the south a slight elevation forms a part- 
ing between its waters and those of the 
minor streams flowing into the Gulf of Mex- 
ico. The main stream flows from the north- 
west corner of the basin along a nearly 
central line more than 4000 miles to the 
Gulf. It is divisible into three sections, 
which differ widely in volume of water and 
other characteristics. (1) The Missouri ex- 
» tends from its source to its junction with the 
I upper Mississippi near St. Louis, more than 
5* 2800 miles. (2) The middle Mississippi ex- 
tends from the mouth of the Missouri to 
; the mouth of the Ohio, 200 miles. (3) The 
I lower Mississippi extends from the mouth 
I of the Ohio at Cairo to the Gulf, 1075 miles. 
! The principal tributaries from the western 
I highland are the Yellowstone, Platte, Ar- 
' kansas, and Red. The volume of the main 
\ stream is almost doubled by the upper Mis- 
j 1 sissippi, which joins it from the north, and 
it receives a still larger accession of water 
from the Ohio, which drains a northeastern 
arm or lobe of the basin. The distance 
in a straight line between the sources of the 
Missouri and the Ohio is nearly 1800 miles, 
and from the extreme northwest corner of 
the basin to the mouth of the river is about 
2000 miles. 

The Missouri River. — The Missouri rises 
at the crest of the Rocky Mountains in 
southwestern Montana by three forks, the 

DR. PHYS. GEOG. — 5 



70 



THE LAND 



longest of which, the Jefferson, is fed by the melting snow 
which fills an old volcanic crater surrounded by peaks 
9000 to 11,000 feet in elevation. It flows through deep 
gorges and glacial lake basins, a mountain torrent de- 
scending 4200 feet in 400 miles, to the point of junction 
with the Madison and Gallatin forks. Thence the Mis- 
souri breaks through the Big Belt Mountains by the 




Fig- 37- — Great Falls of the Missouri. 

"Gate of the Mountains," a canyon 1200 feet deep, passes 
a series of rapids and falls, one of which, the Great Falls, 
has a perpendicular drop of 87 feet, and near Fort Benton, 
600 miles from its source, enters the plateau known as 
the "Great Plains." The Yellowstone escapes from the 
volcanic region of the National Park over falls 350 feet 
high and through a canyon 800 feet deep, and joins the 



THE MISSISSIPPI RIVER SYSTEM 



71 




Missouri 400 miles out 
on the plains. 

From its source to the 
Great Falls the Missouri 
has an average fall of ten 
feet per mile ; from the Falls 
to the mouth of the Yellow- 
stone, two feet four inches 
per mile ; from the Yellow- 
stone to the junction with 
the Mississippi, less than 
one foot per mile. 

The rainfall of the 
Missouri basin is less 
than twenty inches a 
year, and the source of 
water supply is largely 

from the melting SnOWS Fig- 38. —Gorge of the Yellowstone. 

upon the mountains. The volume of water varies greatly, 
being at high water in June about thirty times as great as 
at low water in November. The loss by evaporation is 
so great that the river in summer actually grows smaller 
as it advances across the plains, and it succeeds in dis- 
charging at its mouth only twelve per cent of the total 
yearly rainfall in its basin. As a result of this the river 
is overloaded with sediment, and justifies its name, " Big 
Muddy." 

The Missouri Valley. — Through a great part of its length the valley 
consists of three trenches, one within another. The widest trench, 
or valley proper, five or six miles across, consists of a flat or gently 
sloping plain, covered with the deposits of the river and its tributaries, 
and bordered by bluffs or terraces. Winding through the valley is a 
second trench or bed of the river occupied at high water, and on the 
bottom of this the low water channel swings from side to side. 

The Platte, Arkansas, and Red Rivers. — The Platte and the Arkan- 
sas stretch from the mountains across the dry plains, and are subject 



72 



THE LAND 



to conditions similar to those of the Missouri. Both grow smaller by 
evaporation, and the Platte is at times a mile wide and only six inches 
deep. The Red is far enough south to catch the rains from the Gulf, 
and is less variable in volume. 

The Upper Mississippi rises among the lakes and forests 
of northern Minnesota, at an elevation of about 1 500 feet 
above the sea. It flows by a tortuous course from lake to 
lake, with intervening rapids, about 600 miles to St. Paul ; 
its average fall in this course is about fifteen inches per 




Fig- 39- 



-Flood plain on upper Mississippi River. 
(Near St. Cloud, Minn.) 



mile. From St. Paul to its junction with the Missouri, 660 
miles, its average fall is about five inches per mile. From 
St. Paul to the sharp bend at Muscatine, the valley is gen- 
erally two or three miles wide ; below Muscatine its width 
is seldom less than five miles. 

The upper Mississippi has a basin whose area is only one third that 
of the Missouri, but the rainfall is thirty-five inches, which enables it to 
flow as a strong stream more than half a mile wide at its mouth, and 
to contribute nearly as much water as does the Missouri. It is subject to 
less fluctuation in volume than either the Missouri or the Ohio, but has 



THE MISSISSIPPI RIVER SYSTEM 73 

a period of high water from February to July. It is lowest in Decem- 
ber, but is always navigable without difficulty as far as St. Paul. Its 
banks are marked by many high and picturesque bluffs. 

The Middle Mississippi resembles the upper Mississippi 
in the character of its valley, which is five to seven miles 
wide, and bordered by limestone bluffs. Its channel is 
much divided by islands formed by deposits from the 
muddy water of the Missouri. Its fall is seven and one 
half inches per mile. 

The Ohio. — The basin of the Ohio is less than one half 
as large as that of the Missouri, but the rainfall is forty- 
three inches, and the river discharges about three fourths 
as much water as the Missouri and upper Mississippi com- 
bined. It rises by two nearly equal branches, — the Monon- 
gahela from West Virginia, and the Allegheny from north- 
western Pennsylvania. After a course of about 300 miles 
each from the summit of the Alleghany Mountains, and a 
fall of 1000 feet, they unite at Pittsburg, 700 feet above 
the sea, and nearly 1000 miles from the Mississippi. Be- 
tween Pittsburg and Cairo the river presents a series of 
shoals and rapids separated by reaches or pools in which 
the water is deeper and the fall very slight. At the Louis- 
ville rapids there is a fall of 23 feet in 2.25 miles. 

The valley of the Ohio is deeply cut into the Alleghany plateau, which 
slopes westward from the mountains to Indiana. The valley is usually 
not more than one or two miles wide, and bounded by steep bluffs from 
100 to 400 feet high (see Frontispiece). The river is from a half mile 
to one mile in width, and is bordered by a continuous flood plain. It 
is subject to excessive fluctuations of volume, and at the highest stage 
discharges thirty-four times as much water as at the lowest. The spring 
rains and melting snows of February and March sometimes raise its 
level at Cincinnati seventy feet above low water mark, and the droughts 
of summer sometimes reduce its depth to two or three feet. In the last 
200 miles of its course the bluffs become lower and the valley wider, 
with a corresponding expansion of the flood plain to ten or more miles, 



74 



THE LAND 



SCALE OF MILES 




GULF OF MEXICO 



Fig. 40. —Lower Mississippi flood plain. 



through which the river 
winds in a manner similar 
to that of the lower Missis- 
sippi. The Ohio is gen- 
erally navigable for small 
boats as far as Pittsburg. 

The Lower Missis- 
sippi. — The alluvial 
valley of the lower 
Mississippi from the 
mouth of the Ohio to 
the Gulf is 600 miles 
long, but the distance 
as the river flows is 
nearly twice as great 
(1075 miles). The width 
of the flood plain va- 
ries from 25 to 80 
miles. It is bounded 
on the east by clay 
bluffs 100 to 300 feet 
high. There are also 
bluffs on the west side 
as far down as the Red 
River, but they are not 
so prominent as those 
on the east. 

The course of the main 
channel of the Mississippi 
is near the east bluff as far 
as Memphis, then it crosses 
to the west side of the val- 
ley at Helena, but soon 
crosses again to the east 
bluff, which it strikes at 



THE MISSISSIPPI RIVER SYSTEM 75 

Vicksburg and follows as far as Baton Rouge. The surface of the 
valley is mostly below the level of the river banks, and is traversed by 
an intricate network of side channels and sluggish streams, some of 
which receive water from the Mississippi as well as discharge into it. 
The valley contains four large basins. The St. Francis basin lies 
on the west side of the river, and extends from a point above Cairo 
nearly to the mouth of the Arkansas. The St. Francis River flows 
through it parallel with the Mississippi, receives all the small tributaries 
from the west, and empties into the main stream. The Yazoo basin lies 
on the east side of the valley and is drained by the Yazoo River, which 
begins in a branch leading out of the Mississippi above the mouth of 
the St. Francis, flows near the east bluff, receives all the eastern tribu- 
taries, and joins the main stream near Vicksburg. The Tensas basin 
lies on the west side below the Arkansas, and is drained by the 
Tensas bayou through the Black River into the Red River. Below the 
mouth of the Red lies the basin of the Atchafalaya, a river which re- 
ceives water from both the Red and the Mississippi, and pursues an in- 
dependent course to the Gulf, which it enters 100 miles west of the mouths 
of the larger stream. Thus the alluvial valley is traversed throughout its 
length by a secondary channel on the opposite side from the main stream 
and parallel with it, which receives water both from the main stream and 
from the tributaries, to be discharged at some point farther down. 

The main channel is extremely tortuous, and frequently 
divides into two or more channels, which inclose islands 
or bars. Figure 41 shows its course near Greenville, Miss., 
where the river appears to be wriggling from side to side in 
a series of S curves. On the inside of each bend, and on 
the downstream side of each tongue of land, there is a 
sand bar, while on the opposite side the water is deep and 
swift. The swifter current on the outside of each bend, 
and on the upstream side of each tongue, cuts away the 
bank at those places, while the slower current on the oppo- 
site side, 'by depositing sediment, builds a new bank. Thus 
the tendency of such bends is to grow more crooked and to 
travel downstream. There is a tendency also for the nar- 
row neck of land, which separates one bend from the next, 



7 6 



THE LAND 




Fig. 4i- 



to grow narrower. Finally, 
at some time of high water, 
the river cuts through or 
runs over the neck. The 
slope and fall previously 
distributed throughout the 
long bend are now con- 
centrated in the new and 
short cut, and the current 
there has such a velocity 
as to widen and deepen the 
channel very rapidly, form- 
ing a permanent cut-off. 

Lake Chicot and Lake Lee 
(Fig. 41) are old bends of. the 
river, which have been cut off 
and partly filled with sediment. 
Thus while the river is growing 
more crooked in one place, it 
straightens itself in another, and 
the average sinuosity remains 
about the same. The valley 
abounds in horseshoe- or cres- 
cent-shaped lakes, all of which 
were once portions of the chan- 
nel, and show how the river, in 
the course of ages, has shifted its 
bed from one side of the valley 
to the other, and has occupied at 
some period every portion of it. 

Floods. — As may be in- 
ferred from what - has been 
said of the Missouri and 
Ohio, the lower Mississippi 
is subject to great floods. 



THE MISSISSIPPI RIVER SYSTEM 



77 




which give it more than ten times the volume it has at low 
water. The water rises 53 feet at Cairo, 36 at Memphis, 
48 at Helena, 53 at Vicksburg, and 15 at New Orleans. 
If not artificially re- 
strained, the river at 
high water spreads out 
and covers nearly the 
whole valley from bluff 
to bluff. As the water 
overflows its banks, the 
current is checked rather 
suddenly by its shallow- 
ness and by the willows Fig 42. -Levee, Mississippi River. 

and other vegetation. It consequently drops the larger 
and coarser part of its load of sediment within a mile or 
two of the channel, and builds up its banks higher than 
the general level of the valley floor, forming natural levees. 
The flood water deposits a thin layer of fine mud over the 
whole submerged country, and returns through the nu- 
merous bayous, or side 
channels, to the main 
stream farther down 
the valley. 

Thus the spaces inclosed 
by the network of channels 
become platter-shaped de- 
pressions,, many of which 
are occupied by cypress 
swamps. The natural levees 

upon both sides of the river 
Fig. 43. -Crevasse, Mississippi River. haye been raised by arti . 

ficial embankments of earth designed to prevent- the high water from 
flooding the valley ; but frequently the river breaks through the levee, 
forming a crevasse, which rapidly widens and transmits a raging and 
destructive torrent of water. Below the mouth of the Red the rise of 




y8 THE LAND 

the river is much less because the surplus water escapes through the 
Atchafalaya, which sometimes carries all the water of the Red and one 
third that of the Mississippi. 

The Delta. — At a point 300 miles above the mouth of 
the Mississippi, the Atchafalaya, the first distributary, or 
branch which does not rejoin, leaves the river (Fig. 40), 
and thence carries part of its water to the Gulf. This 
point is the present head of the Mississippi delta, an area 
of 10,000 square miles of lowland and marsh, much of 
which is scarcely above the level of the sea. The Missis- 
sippi River carries to the Gulf a load of fine mud, sufficient 
to cover a square mile about 270 feet deep every year. 
Thus the delta is being gradually extended into the Gulf. 
About fifteen miles from the sea the river divides into three 
arms called "passes," which subdivide into smaller arms, 
each of which has built for itself natural levees which 
appear above the waters of the Gulf as narrow tongues 
of land, the whole forming a tract called "the Goosefoot." 

The mouth of the South Pass is kept open for large vessels by jetties 
or embankments built upon each side in such a manner as to confine and 
quicken the current and compel it to deepen the channel across the bar. 

Other Features. — The alluvial valley of the lower Mississippi is a 
very broad and shallow trench cut through loose sedimentary material, 
chiefly sand and clay. The depth of this material varies from 100 feet at 
Cairo to more than 1000 feet at New Orleans. The width of the river 
channel varies from a half mile to two and a half miles ; the depth of the 
river at low water, from five feet to 150 feet. The fall from Cairo to the 
head of the delta is less than six inches per mile ; through the delta at 
low water it is but a small fraction of an inch per mile. The average vol- 
ume of water discharged is about 600,000 cubic feet per second, and the 
velocity of the current varies from one mile to four miles per hour. The 
lower Mississippi is one of the muddiest rivers in the world, the greater 
part of its sediment being furnished by the Missouri. The increase of 
volume and slope at high water quickens the current and enables it to 
deepen and straighten the channel. At the same time the overflow 
deposits sediment and builds up the general level of the flood plain. 



THE MISSISSIPPI RIVER SYSTEM 79 

At low stages of water the current is feeble and easily deflected by any 
obstruction. Consequently it staggers from side to side under a load 
which it drops at one place and picks up again at another. Thus it 
wriggles about all over the surface of the valley and maintains it, on 
the whole, at about the same width. There is some evidence which 
indicates that it is slowly raising its bed and banks. The final result 
of its work is the extension of its delta out into the Gulf and the build- 
ing up there of a pile of sedimentary strata more than 1000 feet thick. 

Work of the Mississippi System. — The vast system of the 
Mississippi furnishes examples of almost every variety of 
stream and stream work. The head waters of the Missouri 
rush over rapids and cataracts, and are able to corrade 
mountain gorges and canyons. The upper Mississippi is 
busy in draining and slowly destroying a multitude of gla- 
cial lakes. The Ohio has cut a deep trench in the Alle- 
ghany plateau, and is now engaged chiefly in widening it. 
The lower Mississippi has reached its base-level, or the 
lowest level to which its current and load will permit it to 
reduce its bed, and is now sawing sidewise at places into the 
restraining bluffs. The whole system is tearing down the 
land, and carrying it into the Gulf, at a rate which lowers 
the average level of its basin one foot in about 3500 years. 

The lower Mississippi is said to be in its old age because its destruc- 
tive work is nearly accomplished, and it can now do little more than 
transmit the waste material supplied by its tributaries. The Ohio and 
its tributaries have hardly yet reached maturity because they have the 
greater part of their possible task of land degradation still before 
them. The branches of the upper Missouri are infant streams, which 
have just begun the work of tearing down and carrying away the great 
mass of the Rocky Mountains. If nothing interferes, the condition of 
old age already approaching in the lower Ohio will creep up that 
stream and up the Mississippi and the Missouri and their tributaries, 
until the plateaus and the mountains disappear and the Mississippi 
basin is reduced to a low and nearly featureless plain. 

Summary. — The Mississippi system may be taken as 
a typical example of most of the great river systems of 



80 THE LAND 

the world. Its head waters descend from lofty mountains, 
and its middle course is through a region of moderate 
elevation and of large average rainfall. The slope of its 
bed is steep near its source, and decreases more and more 
slowly to its mouth ; that is, its longitudinal profile is con- 
cave to the sky (see Fig. 36). The ramification of its main 
stream into numerous branches, like the limbs, branches, 
and twigs of a spreading tree, the occurrence of rapids, 
cataracts, gorges, and lakes among its head waters, and 
their absence elsewhere, the wide and gently flowing cur- 
rents of its middle course, and the large proportions of its 
alluvial valley and delta are common characteristics of 
great rivers. 

Exercise. — Examine physical maps of the different continents and 
compare the following rivers with the Mississippi, as to area and form 
of basin, length, fall, tributaries, origin of head waters, course, alluvial 
valley, and delta : Amazon, Orinoco, Plata, Hoang, Yangtze, Ganges, 
Amur, Lena, Yenisei, Obi, Volga, Danube, Mackenzie, Yukon. Kongo, 
Niger. 



CHAPTER VI 
THE COLORADO RIVER SYSTEM 

The Colorado River system drains a series of lofty pla- 
teaus surrounded by still loftier mountains. The greater 
part of its basin lies between 5000 and 10,000 feet above the 
sea. Its farthest sources 
are in the mountains of 
western Wyoming, at an 
elevation of 12,000 feet, 
whence the Green River 
flows southward about 
400 miles. In southeast- 
ern Utah the Green is 
joined by the Grand, 
which rises among the 
highest peaks of the Park 
Range in Colorado. The 
junction of these rivers 
forms the Colorado, 
which flows southwest- 
ward 200 miles, then 
turns to the west for 150 
miles, then flows south 
about 300 miles to the 

Gulf Of California. The Fi « 44- -Basin of the Colorado River. 

whole length of the Green-Colorado, by the windings of the 
river, is not far short of 2000 miles. Its principal tribu- 
taries, — - the Grand, San Juan, Little Colorado, and Gila, 
— are all upon its eastern side, its basin is widest near the 

81 




82 



THE LAND 







































FGET 

10 000 






































8 


000 








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6 


000 












^V 


i v ( 


r 






















i 


000 


I 






- a 




^o 












^o 


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2 


ooo 


B 





k / O 


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"SO 


.-R.i 


ve r 




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Fig. 45. —Profiles of the Green-Colorado and Ohio-Mississippi rivers. 

mouth of the river, and in contrast with the wide-spreading, 
symmetrical, elmlike branchings of the Mississippi, the 
map of the Colorado system resembles a bent and broken 
trunk with a few straggling and one-sided limbs. Over 




scale onun.ES 



6' ' ' '5 io 15 20 



Fig. 46. — Part of Green River. 



THE COLORADO RIVER SYSTEM 



83 




Fig. 47. —Canyon of Lodore, by which Green River leaves Browns Valley. 

most of the basin the rainfall is less than ten inches per 
year, and the principal sources of water supply are the rains 
and melting snows upon the mountains which stand along 



8 4 



THE LAND 



its border, the Rocky Mountains on the east and north, 
and the Wasatch on the west. In northern Utah the 
Uinta Mountains project eastward from the Wasatch half- 
way across the basin. 

The Upper Green River. — North of the Uinta Mountains the Green 
River traverses a plateau into which it has cut a broad valley about iooo 
feet deep. On reaching the northern foot of the mountains the river 
enters a canyon which penetrates directly into the range to a point within 
five miles of the crest, then turns abruptly to the east and runs along 
the axis, gradually crossing toward the south. The easterly course of the 
river for forty miles is through a broad valley (Browns Valley) which 
continues to the end of the range. The river, however, does not follow 

this valley around the end of 
the mountain ridge, as it ap- 
parently might, but turns 
sharply to the southwest and 
crosses the range through the 
Canyon of Lodore (Fig. 47). 
On the south side of the moun- 
tains it cuts off the end of a 
projecting plateau at a point 
where a course a few miles 
longer would have carried it 
around that elevation. 

The Terrace Canyons. — Be- 
tween the valley of White 
River (Fig. 53) and the mouth 
of the Grand, a distance of 
about 150 miles, the surface of 
the country is like a staircase 
which has been tipped back- 
ward so that each tread or step 
Fig. 48. - The Terrace Canyons. slopeg up ^^ thg riser next 

below (see Figs. 48 and 53). A person traveling southward parallel 
with Green River ascends a moderate slope for 60 miles and reaches an 
elevation of 3000 feet above the river. At this point the upward slope 
ends in a line of very steep cliffs, which drop down in a few miles nearly 
to the river level. The next sloping step is 25 miles wide and the cliffs 




THE COLORADO RIVER SYSTEM 



85 



at its edge are 2000 feet high. The third step is 50 miles wide, and its 
border cliffs are 1200 feet high. 

Green River cuts across the terraces from north to south directly 
against the slope of the steps, and in doing so forms three canyons, 
each of which is shallow at the upper end and gradually increases in 
depth to the lower end. 

Cataract Canyon. — South 
of these terraces the river 
makes a slight easterly turn 
and by doing so runs into an 
elevated ridge, but before 
reaching its central axis turns 
again westerly and runs out 
of the ridge. In the canyon 
thus formed (Cataract Can- 
yon) the walls increase in 
height to about 2700 feet near 
the middle, then decrease to 
the lower end. In the midst 
of it the Green and Grand 
rivers unite to form the Colo- 
rado at a depth of 1200 feet 
below the general level of the 
country. 

Glen and Marble Canyons. 
— From the mouth of Dirty 
Devil River to the mouth of Fig ' 49 - Marble Canyon ' 

the Paria the Colorado flows through Glen Canyon, which has nearly 
perpendicular walls from 200 to 1600 feet high, carved into a great 
variety of glens, alcoves, and amphitheaters. 

Below Glen Canyon is Marble Canyon, which increases in depth 
from 200 feet at its head to its foot, where the walls are 3500 feet high. 
Its width is about twice its depth, and it has been cut through a bed of 
marble 1000 feet in thickness, which stands in smooth, precipitous cliffs 
on either side. 




Grand Canyon. — At the foot of Marble Canyon, the 
Little Colorado comes in from the east, the main stream 
changes its general direction from southwest to west, and 

DR. PHYS. GEOG. — 6 



THE LAND 



: 



Ml 



m i 



the Grand Canyon begins. This so far 
surpasses all other canyons in magnitude 
as to render them comparatively insig- 
nificant. The river passes through a 
series of plateaus, the surface of which 
lies from 6000 to 8000 feet above the 
sea, by a channel which varies from three- 
quarters of a mile to more than one mile 
in depth. The slope of the river is steep 
and broken by many rapids. The sur- 
face of the plateau is highest near the 
upper end of the canyon, where it stands 
6000 feet above the river; but owing to 
the rapid descent of the river bed, the 
canyon is seldom less than one mile deep 
all the way to its mouth at the Grand 
Wash, where the plateau terminates in a 
line of cliffs facing westward. The pla- 
teaus are about 130 miles in width, but 
the course of the river is so crooked that 
the Grand Canyon is 218 miles long. In 
its upper or eastern portion the walls are 
very irregular and cut by side canyons 
into recesses, alcoves, and amphitheaters. 
It is here a valley from eight to twelve 
miles wide, out of which rise a multitude 
of ridges, spurs, gables, towers, and pin- 
nacles, mountainous in size and endless 
in variety of detail, forms which have 
been carved out of the massive strata of 
the plateau. Through the midst of them, 
but sunk far below, winds the slender 
thread of the river from one fourth to one 



THE COLORADO RIVER SYSTEM 



87 



half mile wide. In the lower or western half, the walls are 
more regular and the canyon is distinctly double, consisting 
of an upper and outer 



canyon five or six miles 
wide, and 2000 feet deep, 
through which winds an 



\S 



Fig. 51. — Section of double canyon. 

inner gorge one or two miles wide and 3000 feet deep. 




Fig. 52.— Grand Canyon at foot of Toroweap, showing double canyon. 

Lower Colorado. — A few miles below the mouth of the Grand Can- 
yon, the Colorado turns abruptly to the south and flows 300 miles (by 
the meanders of the river 500 miles) through a nearly rainless country 
to the Gulf of California. As it approaches the gulf, its flood plain is 
ten or more miles in width, and it resembles the lower Mississippi. 

The Gulf of California once extended more than 100 miles farther to 
the northwest than it now does. The mouth of the Colorado River was 
then on the eastern side of the gulf, but the river extended its delta 
until it formed a barrier which cut off the head of the gulf from the 
main body. The water evaporated from the basin thus formed, and 
now known as the Salton Desert. This is 266 feet below sea level, 
and is sometimes temporarily flooded by the waters of the Colorado. 

Summary. — The characteristics of the Colorado River 
distinguish it above all other rivers in the world. From 
the north side of the Uinta Mountains to the foot of the 



88 



THE LAND 



COLORADO PLATEAUS 




GRAND CANYON 



Fig. 53. — Section along the canyons of the Gre 

Grand Canyon, a distance as the river flows of nearly 1000 
miles, the stream follows a course entirely regardless and 
apparently in defiance of the surface and slope of the 
country. Mountain ranges and massive plateaus stand 
across its path, but they seldom turn it aside. In several 
instances it seems to go out of its way to cut through them 
or to run into a ridge and out again. Not only in particu- 
lar cases, as through the Terrace Canyons, but in the greater 
part of its course, it flows directly opposite to the general 
slope of the country. The elevation of the Colorado pla- 
teaus above the sea is several thousand feet higher than 
that of Browns Valley. As a result of this, the river flows 
for a thousand miles, with trifling exceptions, at the bottom 
of a steep-walled trench, sunk thousands of feet below the 
general level of the country. 

Origin of Canyons. — The first impression made upon 
one who sees these canyons, or pictures of them, is likely 
to be that some force acting from the interior of the earth 
has broken the crust apart and made a great crack, which 
the river had only to follow. But this theory will not bear 
investigation. Cracks in the earth-crust do occur, but 
none have ever been found so crooked as this series of 
canyons. Cracks are almost always accompanied by fault- 
ing, or a displacement of the rock on one side up or down ; 
but in the canyons the strata on one side correspond to 
those on the other, as if they had once extended across 
the chasm. Several faults occur in the Colorado plateau 
region (see F, Fig. 53), but they are not parallel with 



THE COLORADO RIVER SYSTEM 89 




. „„^bmJth PANYON GRAY C. DESOLATION CANYON 

CATARACT CANYON ^TERRACE CANYONS 

,200 100, MILES 



Jolorado River, shortened by omitting bends. 

the river, which runs across as regardless of them as it is 
of mountains. Each tributary river has a canyon of its 
own, and so, too, has each smaller branch. Each canyon 
is adjusted to the size of the stream which flows through 
it, and the level of its bottom is usually adjusted to that of 
the larger canyon into which it empties. Thus, the whole 
tract of plateaus and mountains is divided by a labyrinth 
of ramifying canyons into irregular blocks. Every rod of 
this network, from source to mouth and from top to bottom, 
shows evidence of being the work of running water. The 
conclusion is unavoidable, that the streams have carved 
their own canyons. 

A stream can not begin to cut a valley until it has begun 
to flow along the course of the valley. It begins at the 
top and works downward. It is evident that the Green- 
Colorado River never could have flowed over mountains, 
up slopes, and across plateaus on such a surface as now 
exists along its course. The only solution of the problem 
is found in the supposition that the river established its 
channel when the surface sloped continuously, or nearly 
so, in the direction of its flow, and that it has maintained 
nearly the same general course and level through all the 
subsequent movements of the earth-crust in its basin. It 
has acted very much like the saw in a sawmill, which cuts 
a groove into anything presented to it. The earth-crust 
has been pushed up, arched, and broken, and the blocks 
have been tilted at various angles, while the river system 
has been sawing its canyons. 



90 THE LAND 

The river has been able to corrade thus deeply because (i) its 
slope is steep, through the canyons six to ten feet to the mile, and 
its current swift, (2) it is supplied with a full stream of water from the 
mountains, and (3) it carries a sufficient load of sediment for active 
corrasion without being overloaded and choked. The canyons are nar- 
row, because at elevations below 8000 feet the region is nearly rainless, 
weathering goes on very slowly, and the "tributaries which would cut 
down the walls and widen the valley are few, short, and inconstant. 
The double gorge of the Grand Canyon furnishes striking proof of the 
fact that since the river began to flow the country has stood at a much 
lower level than now. When the wide outer gorge (see Fig. 51) was 
completed, its bottom was' not far above sea level, the river had a 
feeble current and wound from side to side, eating back the cliffs until 
its valley became in some places fifteen miles wide. Then came a 
slow upheaval of the land which gave the river a rapid descent, quick- 
ened its activity, and caused it to cut into the floor of the old valley a 
narrower and deeper canyon. 

Work of the Colorado System. — The Colorado River 
furnishes on the largest scale and in the greatest variety 
examples of the work of a river which drains an elevated 
and arid region. Empowered by its rapid descent, it has 
engraved upon the face of the earth, in plain characters, 
the story of what running water, together with running 
sediment, may accomplish. Its short, steep tributaries, 
flat divides, and narrow canyons are evidences of scant 
rainfall and relatively small water supply. If the rainfall 
in the Colorado basin had been forty inches instead of less 
than ten inches per year, weathering would have been 
rapid and the river would have been much larger, with 
longer and more numerous tributaries. In the time which 
it has taken to carve its narrow canyons, it would have 
worn down the mountains and plateaus, and perhaps filled 
the Gulf of California with their debris, as the Mississippi 
has filled much of the Gulf of Mexico. 

The Colorado has been called a "precocious infant" because although 
it may be called young it has accomplished a great work. Yet it has 



THE COLORADO RIVER SYSTEM 91 

done very little in comparison with what remains for it to do. There are 
other rivers which present characteristics similar to those of the Colo- 
rado, but on a smaller scale. The Rio Grande drains an area of arid 
plateaus lying southeast of the Colorado basin. In passing through 
the ranges of mountains between Presidio and the mouth of the Pecos 
it traverses a series of narrow canyons 1000 to 5000 feet deep and 350 
miles long. The Snake River, rising from the same mountains as 
the Missouri and the Green, flows westward through the lava beds of the 
Columbia plateau by a series of canyons, one of which is fifteen miles 
wide and 4000 feet deep. The rivers which drain the lofty plateaus of 
central Asia to the east and south — the Hoang, Yangtze, Mekong, and 
Brahmaputra — descend to the lowlands through stupendous gorges, 
some of which have never been fully explored. 

Exercise. — Write a comparison of the Mississippi and the Colorado 
in regard to basin, tributaries, divides, fall, current, rapids, cataracts, 
valley, and amount of sediment carried. 



CHAPTER VII 

THE ST. LAWRENCE RIVER SYSTEM 

The St. Lawrence basin may be regarded as a shallow de- 
pression in the long eastward slope of North America and 
as a broad gap in the 'eastern highlands, connecting the 




Fig- 54- — Basin of the St. Lawrence River. 

central plains with the Atlantic coast. Few places in the 
divide which surrounds it are more than 1500 feet above 
the sea, and on the south the divide is in many places less 
than half as high. About one sixth of the basin is covered 
by the five Great Lakes which lie close to its southwestern 
border. The tributary streams, except the Ottawa and 
Saguenay from the north, are short. The level of Lake 
Superior is 602 feet above the sea, and the descent from 
it, through the St. Marys River, to the level of the twin 

92 



THE ST. LAWRENCE RIVER SYSTEM 



93 



lakes, Michigan and Huron (581 feet above sea level), 
is twenty-one feet. Lake Huron is connected by St. 
Clair River and Lake and Detroit River with Lake Erie, 
which lies eight feet lower (573 feet). From Lake Erie 
to Lake Ontario, through the Niagara River, there is 



MILES 
12 15 18 21 24 27 




MILES 500 1000 1500 2000 

Fig. 55. — Profiles of the St. Lawrence and Niagara rivers. 

a drop of 326 feet in thirty miles. The St. Lawrence 
River leaves Lake Ontario at an elevation of 247 feet, and 
with an average fall of one foot per mile reaches sea level 
at Three Rivers, 500 miles from its mouth. Between Lake 
Ontario and the mouth of the Ottawa at Montreal, the 
river is wide, straight, swift, clear, bank-full, without floods 
or flood plain, and with numerous rocky islands and rapids. 
Some of these peculiarities may be accounted for by the 
influence of the lakes. Sediment carried by streams into 
lakes settles there, and the water flows out clear. In the 
absence of sediment, the river lacks the tools necessary for 
corrasion, and makes very little impression upon the bed 
over which it flows : hence the stream channel is but 
slightly depressed below the top of its banks. Any tem- 
porary excess of water supply is spread out on the broad 
surface of the lakes, and has no appreciable effect in rais- 
ing their level ; consequently floods can not occur in the 
river below. From Montreal to the mouth the current is 



94 



THE LAND 



SCALE OF MILES 




^ r^~T^^ 



Fig 56 — Laurentian channel. 



affected by the ocean tides. Below Three Rivers the 
river is 1000 feet or more in depth, widening into a great 

estuary, the Gulf of St. 
Lawrence. A channel 
about 2000 feet deep 
extends along the bot- 
tom of the gulf, and 
300 miles out to sea. 
The course of the 
St. Lawrence, includ- 
ing the Great Lakes, 
presents striking pecul- 
iarities. In its upper 
part it resembles a 
stream which has been 
obstructed by a series 
of dams, above each of which the water is held back in a 
great pond or reservoir. In its lower part it is not now 
a river, but an arm of 
the sea which extends 
up the valley 500 miles. 
When the river made 
this valley and the chan- 
nel upon the bottom of 
the gulf, it must have 
had in this part of its 
course a current of con- 
siderable velocity and it 
must have carried sedi- 
ment. These conditions 
point back to a time 
when the St. Lawrence system did not contain many lakes 
and when its basin stood at an elevation of 2000 feet or 




10 20 30 40 50 



Fig- 57. — Hudson channel. 



THE ST. LAWRENCE RIVER SYSTEM 



95 



more above its present level. Since that time the course 
of the old Laurentian river has been obstructed by a num- 
ber of dams above which the waters of the Great Lakes 
are held up to their present levels, and the land has sub- 
sided so as to let the sea into the lower valley, converting it 
into the present broad 
gulf. 

Drowned Valleys. — The 
lower portions of stream 
valleys which have sunk 
below sea level are called 
drowned valleys. _ The 
lower St. Lawrence is per- 
haps the greatest example 
of a drowned valley in the 
world, but many other rivers 
are in the same condition. 
The old channel of the 
Hudson River may be 
traced upon the sea bot- 
tom about 125 miles be- 
yond its present mouth 
(Fig. 57), and its valley is 
drowned as far up as Troy, 
150 miles. The sea ex- 
tends up the Delaware 
River to Trenton, and 
Chesapeake Bay with its 
many arms is the drowned 
valleys of the Susquehanna 
and its former tributaries 
(Fig. 58). Many of the 
most famous harbors in the world, as San Francisco Bay, Puget Sound, 
the estuaries of the Thames and the Mersey, and the Scottish firths, 
are drowned valleys. 

The Niagara River and Falls. — The strip of country be- 
tween Lake Erie and Lake Ontario, twenty-five miles wide, 




.30,. 40 50 



Fig 58- — Delaware and Susquehanna channels. 



9 6 



THE LAND 




consists of two plains lying at different levels. The upper 
plain extends from Lake Erie northward eighteen miles to 

the edge of an escarp- 
ment or cliff, where the 
surface drops steeply 
down 200 feet to the 
level of the lower plain, 
which borders Lake On- 
tario. Both plains are 
underlain by strata of 
sandstone, limestone, and 
shale, which are not quite 
horizontal, but slope 
southward about thirty- 
five feet to the mile. The 
arrangement is such that 
the strata outcrop on the 
surface in the following 
order from the shore of 
Lake Erie: (1) hard 
limestone (Corniferous), 
(2) soft shales (Salina), 
Fig. 59. - Niagara plains (3) hard, thick -bedded 

limestone (Niagara), extending to the edge of the escarp- 
ment, (4) soft shales and thin-bedded limestones (Clinton) 
forming the lower part of the cliff and the surface of the 



N I A G 4 fi| A L ! M H S T O N 



ALES 



L t M E S T 0/ 




Buffalo 



LAKE ERIE 



Fig. 60. — Section of Niagara plains. 

lower plain (Figs. 59, 60). The Niagara River flows 
across these two plains from one lake to the other. Its 
course is quite direct, so that it makes the whole descent 



THE ST. LAWRENCE RIVER SYSTEM 97 

of 326 feet in about thirty miles. In the first thirteen 
miles the river is five to twenty feet deep and about one 
mile wide except where it is divided by Grand Island. 
The banks are low, and the current moderate. The stream 
resembles the St. Lawrence below Lake Ontario, and has 
corraded its channel to a very slight depth. Where it 
reaches the Niagara limestone, rapids begin, and after 
rushing down a slope of fifty-three feet in half a mile, 
the river drops perpendicularly 160 feet into a narrow 
gorge, which extends seven miles to the escarpment, and 
there opens out upon the lower plain. The width of the 
gorge varies from 600 to 1200 feet, and its nearly perpen- 
dicular walls rise 200 feet above the water. The slope is 
very steep and the water rushes through the narrow chan- 

ABOVE THE FALLS 

Fig. 61. — Cross sections of the Niagara. 

nel in a succession of boiling rapids (Fig. ||| 
62). Midway in the length of the gorge is 
the Whirlpool, where an expansion and a bend cause the 
current to circle around in a complete loop. After passing 
under itself, it escapes at right angles to the course of the 
incoming stream. The peculiar features of the river which 
demand explanation are the falls, the gorge, and the sud- 
den change in the character of the valley from wide and 
shallow to narrow and deep. 

In the walls of the gorge, strata of varying composition and hardness 
are exposed. At the top the Niagara limestone forms a bold, perpen- 
dicular face about fifty feet high. Below, a series of soft Clinton shales 
are weathered into a steep slope. About midway of the height harder 
strata of Clinton limestone form a low cliff, below which soft shales and 
sandstones again form a steep slope to the water's edge. The strata 
on one side of the gorge correspond exactly in character and position 
with those on the other side, so that there is no suggestion of a fault or 
displacement. At the falls the same strata occur in the same order. 



9 8 



THE LAND 




Fig. 62. ^Niagara Gorge. 




Fig. 63. — Niagara Falls. 



THE ST. LAWRENCE RIVER SYSTEM 



99 







The upper (southern) part of the river joins the gorge at a right angle 

(Fig. 59), so that the portion of the stream which forms the American 

Fall drops over the side of 

the gorge, while the larger 

Canadian or Horseshoe Fall 

plunges in at the end, the two 

streams being separated by 

Goat Island (Fig. 63). At the 

brink of both falls the Niagara 

limestone overhangs like the 

cornice of a house, so that a 

considerable space intervenes 

between the falling water and H - 

the face of the precipice (Fig. 

64). This space behind the Fi S- 64. -Section of Niagara Falls. 

American Fall is called the "Cave of the Winds," and is large enough 

to admit visitors. At the 
foot of the American Fall 
lie many great blocks of 
limestone which have fal- 
len from above (Fig. 65). 
From the brink of the 
Horseshoe Fall similar 
blocks many yards in area 
have been observed to fall 
from time to time. A com- 
parison of the positions of 
the brink of the Horseshoe 
Fall as determined by sur- 
veys made in 1842 and in 
1 890 (Fig. 66) shows that the 
fall has moved upstream on 
an average five feet a year. 

Method of Recession. 

— The water passing 

Fig. 65. - The American Fall. over the brink of the 

falls strikes the bottom with great force, and boils upward 
again, while a portion of it constantly splashes back against 
LoFC. 




100 



THE LAND 




the face of the precipice behind. In winter blocks of ice 
are hurled back against the wall, and add to the destruc- 
tive effect of the splashing water. The soft shales are 
worn away, leaving the limestone above unsupported, which 

sooner or later fails by its own 
weight. In the Horseshoe 
Fall the force of the water is 
sufficient to toss the fallen 
blocks about, and to use them 
as tools to undermine the 
limestone still farther. The 
American Fall is too feeble 
to break up and carry away 
the blocks. Consequently 
they have accumulated and 

Fig. 66. —Map of Horseshoe Fall. ,, . . 

now protect the precipice 
somewhat from further attack. From all these facts it 
seems evident that at some time in the past the Niagara 
River began to fall over the escarpment (Fig. 59), and that, 
by the processes just described, the falls have traveled 
upstream to their present position. The Niagara gorge, 
therefore, has not been made by downward corrasion, for 
which the stream, on account of the absence of sediment, 
is poorly fitted ; but it is the result of excavation and under- 
mining by the falls. This work has been made possible 
by the position of the hard Niagara limestone on top, and 
the softer strata beneath, and by the fact that the water 
carries little sediment. If the rock in the bed of the river 
above the falls were softer, or if the river carried sediment, 
it would corrade downward and reduce the height of the 
fall by beveling off its edge. When the falls have receded 
to a point where the soft Salina shales are on top, and the 
Niagara limestone at the bottom, they will change from a 



THE ST. LAWRENCE RIVER SYSTEM ioi 

perpendicular cataract to a succession of rapids. A harder 
layer on top and a softer layer beneath are necessary con- 
ditions for the maintenance of a perpendicular fall. Such 
conditions occur frequently, and on almost any stream may 
be found falls which reproduce on a small scale the over- 
hanging ledge, the deep pool, and the gorge which exist at 
Niagara in such magnificent proportions. 

The St. Lawrence basin has had a long and eventful 
history. The river was once mature and had a bed which 
sloped continuously in a curve concave to the sky, like the 
Ohio and Mississippi. By processes which will be de- 
scribed in Chapters X and XI, it has been rejuvenated, or 
compelled to begin again the work of eroding its basin and 
grading its channel. It is now an example of a ponded 
stream, which, by reason of the lakes in its course, is almost 
deprived of sediment and hence of the ordinary means of 
stream corrasion. It is cutting down its bed very slowly, 
except in the Niagara section, where peculiar conditions 
may enable it, in time, to extend its gorge back to Lake 
Erie. That lake will then be drained and its bed will be 
traversed by a river which, by deepening its channel, will 
drain in turn the three upper lakes. 

Exercise. — Using any available source of information, learn the 
characteristics of the Nile River and compare it with the type rivers 
described in Chaps. V, VI, and VII. In what respects does it resemble 
the Mississippi? the Colorado? the St. Lawrence? In the same way 
study the Rhine, the Zambezi, the Indus, and the Euphrates. 



CHAPTER VIII 
UNDERGROUND WATERS 

Ground-water. — Some portion of the rain which falls 
upon the surface of the land sinks into the ground. The 
quantity varies with the steepness of the slope, the cover- 
ing of vegetation, and the porosity of the rock. Fine clay 
and compact limestone absorb water, but permit very little 
to pass through. Sand and coarse sandstone absorb rather 
less water than clay, but transmit it quite freely. Any rock 
which is traversed by joints and cracks, as is usually the 
case in nature, allows the rainfall to penetrate the crust of 
the earth. Some regions of limestones and lavas are so 
broken up by fissures that there are no surface streams, 
the entire drainage being through underground channels. 
Ground-water is continually rising by capillary attraction 
through the soil and keeps growing plants alive in dry 
weather. It is also the source of supply for wells and 
springs. If a well is bored to a depth below the level at 
which the ground is saturated with water it fills up to that 

level. If the water level 
outcrops on the surface, 
a spring occurs at that 
point. 




In Fig. 67 the rain falling 

on the surface HT penetrates 

through the sand until it 

lg 67 ' reaches the surface of the clay 

beneath, and moves slowly toward its lowest point S. But it stands 

higher in the sand than the level of the top of the clay, because a certain 

pressure is necessary to overcome friction and force the water through 

102 



UNDERGROUND WATERS 



103 



the sand. The lowest level of ground- water is at a height where the 
resistance due to friction just counterbalances the pressure due to the 
accumulated water. Since the friction increases with the distance which 
the water has to flow through the sand to its point of escape, it will 
hold the water up to a higher level below T than below H. There will 
be a spring at S, and a well sunk at W down to g will strike water. 
Both spring and well will be unfailing if the rainfall is sufficient to 
supply the outflow from them. If a permeable stratum, as gravel, lies 
below an impermeable stratum, as clay, and receives rain upon its 
outcropping surface, as at O, it may become filled with water up to the 
level of O. Then if a well starting at a lower level, as at A or B, 
is sunk until it taps the water-bearing gravel, the water will rise above 
the mouth of the opening, and a flowing or artesian well will be ob- 
tained. In a boring at B the pressure may be sufficient to raise the 
water to the top of a house or to make a fountain. 

The ground-water everywhere tends to flow or creep slowly toward 
the valleys, where it accu- 
mulates or feeds the surface 
streams. In regions of small 
rainfall and deep, permeable 
soil the greater part of the 
drainage may take place 
through the mantle rock in- 
stead of on its surface. By 
digging in the bottom of a 
dry stream bed, a good sup- 
ply of water may often be 
found, and a dam sunk to 
the proper depth may force 
the hidden stream to rise 
to the surface. Streams 
sometimes increase in vol- 
ume more rapidly than can be 
accounted for by their visible 
tributaries. In such cases 
they receive additions from 
the percolating ground-water. 




Fig. 68. — Stream flowing from a cave. 
(Donaldson's Cave, Lawrence County, Ind.) 



Underground Streams. — In some limestone regions the 
drainage is wholly subterranean and the earth-crust is 



ic>4 



THE LAND 



honeycombed with tortuous passages and tunnels which 
frequently widen into large and lofty chambers or caves. 
The surface of such a region is pitted with funnel-shaped 




Fig. 69. — Section of caves. 

depressions or sinkholes which have no outlet except at the 
bottom. In some cases a stream enters an opening in the 
side of a cliff or hill, and after flowing some distance 
underground reappears upon the surface. Many surface 

streams in limestone re- 
gions flow from caves 
(Fig. 68). 

Caverns. — The rain- 
water percolates through 
the soil, enters the small 
crevices and joints of 
the limestone, and by 
reason of the carbon di- 
oxide which it contains, 
is able to dissolve the 
rock and gradually to 
enlarge the passage. It 
often follows some plane 

of Stratification, hollow- 
Fig-. 70. —Natural Bridge, Virginia, jng put large, irregular 
rooms along that level, and, finding its way to a lower 
level, repeats the process there. The result is a cave in 
two or more stories, connected by numerous passages. 




UNDERGROUND WATERS 



105 



In places the intervening floor breaks down and a lofty 
hall is opened from top to bottom. The place where the 
roof of a cave has fallen in is marked upon the surface 
by a sinkhole or inclosed valley without visible outlet. 
Where the roof of a large tunnel has fallen in, a portion 
may remain standing and form a natural bridge which 
spans the now open valley. The Natural Bridge, in Vir- 
ginia, was formed in this manner. 

Where water carrying lime in solution drips from the roof of a 
cave, it may evaporate, or lose some of its carbon dioxide, or both, and 
thus becoming incapable of 
holding the lime, deposit it 
in a long, pendant stalactite, 
like an icicle. At the point 
where the dripping water 
strikes the floor, more lime is 
deposited and a slender, co- 
lumnar stalagmite is built up 
to meet the stalactite. Thus 
columns, statues, " curtains," 
"altars," " organs," and Qther 
strange and beautiful forms 
are added to the characteristic 
scenery of caves. Mammoth 
Cave in Kentucky, Wyandotte 
and Marengo caves in Indiana, and the Luray Cavern in Virginia, are 
among the most famous and extensive in the world. Wyandotte Cave 
has a measured length of more than four miles, and contains one room 
210 feet long, 90 feet wide, and 65 feet high. 

Mineral Springs. — Water which percolates a consider- 
able distance through the earth-crust meets with a variety 
of minerals which it dissolves and transports to the surface, 
where it emerges as a mineral spring. The nature and 
quantity of the mineral matter held in solution vary with 
the character of the rocks traversed and the temperature 
of the water. 




Fig. 71. 



- Stalactites and stalagmites. 
(Marengo Cave, Ind.) 



DR. PHYS. GEOG. 



io6 



THE LAND 



Many springs of moderate depth and temperature form deposits 
of lime, iron, or other minerals about their mouths, but springs of 
hot water in volcanic regions bring to the surface vast quantities of 
silica which contribute to the formation of extensive masses of rock. 
Hot springs sometimes take the form of geysers, from which, at regular 
intervals,, the water spouts to a great height. Old Faithful, in the Yel- 
lowstone Park, throws a stream of hot water 150 feet high about once 




Fig. 72. — Hot spring terraces, Yellowstone Park. 

an hour (Fig. 73). These periodic outbursts are due to the gradual 
accumulation and final explosion of steam at great depths. 

The Work of Ground-water contributes to the same end 
as that of surface water. It dissolves and eats away the 
substance of the earth-crust and transports it, in some 
cases to higher levels, but finally, by one route or an- 
other, to the sea. Its channels take the form of covered 
tunnels or caves, but these are often changed into valleys 
by the caving in of the roof. It extends the processes of 



UNDERGROUND WATERS 



107 



weathering and erosion to 
indefinite depths, prepares 
the rock for more rapid at- 
tack by surface agents, and 
plays an important part in 
the tearing down and re- 
moval of the land. 

Realistic Exercises. — Men en- 
gaged in sinking wells can fur- 
nish much information concerning 
the ground-water of any locality. 
The student should investigate the 
depth of wells in his neighbor- 
hood, the materials through which 
they pass, and in which water is 
found ; the source, quantity, and 
permanence of the supply ; the 
quality and temperature of the 
water ; the levels at which springs 
occur ; the deposits, if any, at their 
mouths ; and the nature and ex- 
tent of caves, if any exist. Mines, 
quarries, and other excavations 
often show the penetration of 
ground - water and the streams 
which traverse the joints and fis- 
sures of the rock. 




Fig 73 — Old Faithful. 



CHAPTER IX 

GLACIERS 

Upon the tops of mountains and in the polar regions 
upon lower land, most of the moisture which falls from 
the clouds is in the form of snow. If the quantity which 
falls is greater than the quantity which melts and evapo- 
rates, the difference remains from year to year, and the 

ground is always cov- 
ered with a mantle of 
snow. The line above 
which snow is always 
present is called the 
snow line. Its height 
above the sea is great- 
est near the equator 
and in regions of dry 
climate, and least near 
the poles and in re- 
gions of moist climate, varying from 18,000 feet to near 
sea level. On mountain tops snow is blown off the 
peaks and slides down the slopes until it accumulates in 
the valleys to the depth of hundreds of feet. In the 
summer part of the snow is melted by day and frozen 
again at night, rain occasionally falls upon it, and it 
changes from dry, loose snow to a coherent mass, half 
snow and half ice, called neve. The pressure of the upper 
layers upon those below consolidates them and finally 
changes the neve into clear, solid ice. It is the same proc- 
ess of thawing, wetting, freezing, and pressure by which 

108 



• . ' , . 






.. 


frife^l 




• 




\\L '"H 


k\h 


iiiiMTriy^wV *fr 








E^S^' ~~nir m-&> 


.., 1 , 



Fig 74. — Snow-capped mountains. 
(Mont Blanc, Switzerland.) 



GLACIERS 109 

boys make hard, icy snowballs. When the pile of ice, 
neve, and snow becomes deep enough, it begins to spread 
out at the bottom under the pressure of its own weight. A 
basin filled with such material overflows by a stream of ice, 
somewhat as a basin filled with water overflows by a river. 
Ice thus formed from snow instead of water is called gla- 
cial ice ; and any large mass of it is called a glacier. 

Alpine Glaciers. — Glaciers were first studied in the Alps, and those 
mountains still offer to the tourist and student one of the richest and 
most accessible fields for glacial observation. The snow line on the 
Alps lies at a height of about 8500 feet, and the longest glaciers descend 
in the course of ten or fifteen miles to the 4000-foot level. They are 
essentially rivers of ice, each of which conforms to the windings and 
irregularities of its own valley. The rate of motion is seldom more than 
two feet a day, or more than 250 to 500 feet a year, and therefore quite 
imperceptible to ordinary observation. If, however, a row of stakes is 
set across the ice in a straight line with stakes on the banks, the line 
will gradually become more and more convex downstream and the 
rate of movement of each stake may be measured. By this method it 
has been discovered that the motion is more rapid in the middle than 
at the sides, in summer than in winter, by day than by 
night, on steep than on gentle slopes, and in the narrow 

than in the wider parts of the val- — 

ley. The ice does not fill every > ! / / / 

nook and recess of the valley or 
set back into side ravines as water 



4 1 i j 









£"•■-< 


^^--* 






-Vr 


V 


^-«-- j 


y 






1 





would. If Stakes are driven into Movement at side of Movement on surface 



the side of the glacier in a vertical 



a glacier. of a glacier 



row, after some weeks the line will lg ' 75 

be found to incline downstream (Fig. 75), showing that the upper layers 

move faster than the lower. 

Realistic Exercise. — In a long box or trough place a mass of some 
plastic substance like pitch, tar, shoemaker's wax, or asphalt. Stick a 
row of upright pins in a straight line across it and set the box in an 
inclined position. If the material is kept moderately warm, it will flow 
slowly downward, and after a few days the row of pins will be found to 
be convex and inclined downstream. Why ? 



HO THE LAND 

Crevasses. — The surface of a glacier is traversed in 
various directions by cracks called crevasses. One set 
extends from each side toward the center diagonally up- 
stream. These are due to the unequal rates of motion. 
The ice in the center moves faster, while that at the 
sides drags against the valley walls, and the ice is pulled 
in two, or breaks at right angles to the direction of the 
strain (Fig. j6). In passing around a bend the ice upon 
the outer side is put upon the stretch, and crevasses 
appear which often close up again after the bend is 
passed. At points where the valley bottom is convex 

or the angle of slope 
increases abruptly, 




Diagonal crevasses. Longitudinal section of an ice fall. Cross section of a glacier. 

Fig. 76. 

the ice becomes deeply crevassed (Fig. j6). Such places 
correspond to ripples in a river, which remain stationary 
while the water moves on. 

Wherever the ice is suddenly subjected to a pulling or stretching 
strain, it breaks readily like a brittle body, and in many cases it becomes 
so broken by a maze of cracks that it resembles a heap of sharp, angular 
blocks. Where the stream moves on more smoothly, many of the cre- 
vasses close up again, and their sides unite so completely that all trace 
of the break disappears. If a crevasse remains open long, the warm 
air gains access to its surfaces, which melt unevenly, and when the sides 
come together again, they do not fit and the break remains unhealed. 
Ice which has been extensively crevassed never fully recovers its former 
solidity and smoothness. 

Causes of Glacial Motion. — To discover how a rigid, 
brittle body like ice can be squeezed out from under the 
weisrht of its own mass and can then move clown a wind- 



GLACIERS 



III 




Fig- 77- — Davidson Glacier, Alaska. 

ing valley in conformity with its varying direction, width, 
and slope is a problem of great difficulty. 

Plasticity. — While ice is very brittle under sudden strain, it is 
slightly plastic, and will stretch or bend or flow like very stiff molasses 
candy without breaking, if it is only given sufficient time. 

Breaking, Pressure Melting, and Regelation. — The readiness with 
which ice breaks under small strains and its cracks are again healed 
has been already described. When water freezes, it expands, as broken 
pitchers and burst pipes testify every winter. Conversely, when ice 
is compressed it melts. If two blocks of ice with dry surfaces are 
pressed together, slight melting occurs ; when the pressure ceases, the 
water thus formed freezes and the blocks are cemented together. (Try 
the experiment.) This process is called regelation (freezing again). 
The consolidation of snow into ice and the movements of the ice may be 
accounted for by the fact that breaking, pressure melting, and regelation 
are constantly going on. 

Realistic Exercises. — Suspend a lump of ice weighing about twenty 
pounds in a loop of wire. The wire will slowly cut into the ice and 
pass completely through it, but the cut will heal as fast as made, and 
only a layer of air bubbles will remain to show where it was. The ice 
is melted above the wire by pressure and freezes again below it. 

Fill a strong iron box or cylinder with damp snow or small pieces of 
ice and subject them to great pressure under a screw or lever press ; 
they will be consolidated into one mass having the form of the box. 



I 12 



THE LAND 



Melting and Expansion by Freezing. — A glacier moves more rapidly 
in summer than in winter, by day than by night, and in the warmer 
region near its lower end than in the colder region near its source. 
The fact that wherever and whenever there is the most water in the 
ice, it moves fastest, indicates that its motion is due partly to melting. 
Also whenever the water formed by melting freezes again, it expands 
and tends to push the whole mass down the slope. 




Fig. 78. — Aletsch Glacier, Switzerland. 

The causes and methods of glacial motion seem to be 
complex. The principal forces at work are gravity, heat, 
and expansion by regelation. It is impossible to deter- 
mine just how much each contributes to the result. The 
whole truth can not be expressed by such a simple state- 
ment as that a glacier slides or flows or creeps down 
its valley. It probably moves by sliding, flowing, and 
creeping. 



GLACIERS 113 

Ablation. — Throughout the length of a glacier the ice 
is disappearing more or less rapidly by evaporation and 
melting, but, of course, most rapidly in the warmer region 
toward its lower end. On a clear summer day the sur- 
face of the ice is traversed by streams of water which 
unite into drainage systems similar to those upon the 
land. After a longer or shorter course they usually drop 
into some crevasse and disappear in the depths below. 
Around these cascades the ice melts more rapidly, and a 
cylindrical well (inoulin) is formed, extending downward out 
of sight. The melting and evaporation are sometimes suffi- 
ciently rapid to lower the general surface of the ice as much 
as a foot in one day. The glacier finally reaches a point 
in its course where the ablation or destruction of ice equals 
the supply brought down, and the glacier comes to an end. 

At this point a stream of yellowish or milky water issues from the 
mouth of a cave or tunnel in the ice and carries away the whole drain- 
age of the valley above. It is as if a long block of ice were pushed 
toward a hot stove at such a rate that it melts as fast as it comes. 
The ice as a whole is moving forward, but the end remains at nearly 
the same point. The end of a glacier is not strictly stationary, but 
retreats or advances with changes of season and climate. 

Transportation. — An Alpine glacier carries upon its 
surface and in its substance a large quantity of rock debris 
which it has gathered from the sides and bottom of its val- 
ley. On the steep slopes of the mountains weathering 
goes on rapidly, and great avalanches, or slides of snow, 
rock, and earth, descend upon the surface of the ice stream. 
This rock and earth are piled near the margin in a long 
ridge called a marginal moraine. When a tributary joins 
the main glacier the united marginal moraines continue 
down the central part of the combined glacier as a medial 
moraine. These piles of rock and dirt protect the ice 
beneath them from melting, so that by the ablation of the 



ii4 



THE LAND 



bare ice they may come to lie upon a ridge a hundred 
or more feet high. The morainic material rolls down the 
sides of this ridge and spreads out over a wider band, so 




The long undulating arrow follows the line of most rapid motion 
of "Mer de Glace" in the Alps. The amount of movement of the 
surface of the glacier - in inches, per 24 hours in summer- is also 
indicated. 

Fig. 79- 

that at the lower end of the glacier the whole surface of 
the ice is often buried under a mass of gravel and boulders. 
A large amount of rock debris, known as ground mo mine, 
accumulates at the bottom of the glacier and is pushed and 
dragged along with it. It is largely composed of clay, sand, 




Fig. 8o. — Terminal moraine. 
(Middle Blase Dale Glacier, Disco I., Greenland.) 

and gravel. The whole mass of rock and earth carried by 
a glacier upon its surface, in its substance, and at its bottom 
is called glacial drift, and by the final destruction of the ice 



GLACIERS 



115 



it is dumped at the lower end in a confused heap known 
as a terminal moraine. 

Abrasion. — Pure ice moving over a rock surface would 
probably do little more than sweep away loose material ; 
but a mass of ice a thousand feet thick, having sand, gravel, 
and boulders frozen into its bottom, acts like a flexible rasp 
which fits the irregularities of its bed and abrades or wears 
it away in a peculiar and striking manner. All sharp 
angles and corners are rubbed off. The softer portions of 
the bed rock are scooped out into hollows and the harder 
portions are left projecting ; but all the slopes and outlines 
are smoothed and rounded. 

If the bed rock is hard and fine-grained, it may be polished as finely 
as any marble or granite monument. More than this, the glacier leaves 
its signature upon the rock in the form 
of parallel strics, or scratches, from the 
finest hair lines to grooves a foot or two 
deep (Fig. 96). Pebbles and grains 
of sand, grinding along over the rock 
floor under the weight of the ice above, 
wear away the surface and leave scratches 
all running in the same direction. A 
rock surface which has been thus planed, 
polished, and striated is said to be gla- 
ciated. The pebbles and boulders them- 
selves are subjected to the same process, 
and every terminal moraine contains 
thousands of them which present one or 
more glaciated faces. The general effect 
of a glacier upon its valley is to deepen it, to change its cross section 
from a V-shape to a U-shape, and to leave it with a gently undulating 
surface more or less covered with glacial drift. 

Glacial Drift is distinguished from all other deposits by 
well-marked characteristics. (1) Where it has not been 
redistributed by the water flowing from the melting ice, 
it is unassorted and unstratificd. All kinds and sizes of 




Fig. 81. — Glaciated boulder. 



n6 



THE LAND 



sediment are mixed up together higgledy-piggledy. (2) 
The ground moraine is largely a tough clay as full of 
gravel stones as a pudding is of plums, and containing 




Fig. 82. — Map of Muir Glacier. 

glaciated boulders of all sizes. It is called till or boul- 
der clay. (3) The terminal moraine is likely to contain 
more sand and gravel than clay, and any number of large 
boulders of every variety of rock existing along the course 



GLACIERS 



117 



of the glacier which brought them. The stones are gener- 
ally angular or subangular, and may be glaciated on one or 
more sides, but are not smoothly rounded, like water-worn 
pebbles. (4) Glacial drift is largely composed of foreign 
material, that is, of rock fragments which have come from 
a distance and are unlike the bed rock upon which they lie. 

Exercise. — Write a comparison of an Alpine glacier and a river in 
regard to origin, course, movement, transportation, corrasion, deposits, 
and work accomplished. 

"The track of a glacier is as unmistakable as the track of a man or 
a bear." If a glacier should entirely disappear by ablation, what 
evidences of its former existence would remain ? 




Fig. 83. — Muir Glacier, showing ice wall. 

Alaskan Glaciers. — Some of the glaciers which descend 
the mountainous coast of Alaska are different in form from 
any known elsewhere. The Muir Glacier (see Figs. 82 and 
83) is fed by twenty or more ice streams, which descend 



n8 



THE LAND 



into and fill an amphitheater thirty to forty miles in diam- 
eter. The medial moraines, marking the lines of flow, 
converge toward a single outlet about a mile wide, through 
which the surplus left from ablation, the drainage of 800 
square miles of snow field, escapes into an arm of the sea. 
The ice stream ends in a jagged wall not far from 1000 










"=&■ =.-!*«.-- 




Fig. 84. —Map of Malaspina Glacier. 

feet high and standing 200 feet above the water. Large 
masses break off from this cliff and fall into the water 
with a loud roar. Other large masses are loosened from 
the foot of the cliff beneath the water and rise to the 
surface with violent splashing. Thus a continuous proces- 
sion of icebergs float away and melt in the sea. 



GLACIERS 



119 



The Malaspina Glacier, at the foot of the Mt. St. Elias 
range, is in form the reverse of the Muir (see Figs. 84 and 
82). Many separate streams from the mountain valleys 
unite into a plateau or lake of ice which spreads out to a 
width of fifty miles. The 
ice front extends along 
or near the shore of the 
ocean for a distance of 
seventy miles. The sur- 
face of the glacier is un- 
dulating, like the western 
prairies, and in the cen- 
tral portion is mostly free 
from moraines and dirt, 
but broken by thousands 
of crevasses. The outer 
edge of this ice sheet 
seems to have been for 
a long time stagnant and 
has become covered by 
a thick coating of sand 
and gravel derived from 
the moraines. Upon this 
soil a dense growth of 
trees and shrubs has 
sprung up, forming a for- 
est under which the ice is, 
in places, 1000 feet thick. 

The Greenland Ice Cap. — Greenland is a plateau about 
1500 miles long and 800 miles wide in its widest part. 
Two thirds of its surface is buried beneath a sheet of per- 
petual snow and ice. The general elevation of the ice 
plateau is 7000 to 8000 feet in the central area, gradually 




100 200 300 400 



Fig. 85. —Map of the Greenland ice cap. 



120 



THE LAND 



decreasing to 2000 or 3000 feet toward the coast, giving 
to the island a surface form like that of a loaf of bread, 
gently rounded in the middle and steeply sloping at the 
edges. Beyond 50 or 75 miles from the coast no mountain 
peak, rocky islet, or other sign of land rises above the sea 
of neve. The white, featureless expanse is unbroken by 
crevasses or water courses and unstained by dirt or dust. 
The whole mass seems to be moving outward in all 
directions, and, as it approaches the coast, becomes broken 
by projecting peaks of rock and extensively crevassed. 

Its edge is divided 
into numerous long 
tongues which es- 
cape down the nar- 
row valleys to the 
sea. The largest of 
these yet described 
forms the Humboldt 
Glacier, which ad- 
vances into the sea 
Fig. se. -iceberg. with a wall 60 miles 

long and 200 to 300 feet above the water. From the va- 
rious projecting tongues of ice innumerable bergs break 
away and crowd the adjacent waters. The rate of motion 
in the Greenland glaciers sometimes reaches 50 or 100 
feet per day. 

The Antarctic Ice Cap. — The region around the south 
pole as far as latitude yo° seems to be covered with an 
ice cap similar to that of Greenland, but of vastly greater 
extent. Explorers sailing in that direction are stopped by 
an unbroken wall of ice, 200 to 300 feet high, from which 
flat-topped bergs (Fig. 87), often half a mile in breadth, 
break off and float away. The area included within this 




GLACIERS 



121 



ice wall is about 4,000,000 square miles, or larger than the 
whole of Europe. The neve fields, if not continuous over 
the whole region, must be very extensive and moving out- 



■ 


SB 


m 




--»»—. -' J , ' ' .'; 


"'■ 



Fig. 87. — Antarctic iceberg. 

ward in all directions to supply the quantity of ice which 
is discharged. 

Continental Glaciers. — -Glaciers which are not confined 
to valleys but spread over wide tracts of country, like the 
ice caps of Greenland and the Antarctic region, are called 
continental, and are the only surviving representatives of 
vast ice sheets which once covered a large part of North 
America and Europe. 



DR. PHYS. GEOG. 



CHAPTER X 

THE DRIFT SHEET OF NORTH AMERICA 




Fig. 88. — Boulders from a terminal moraine. 

(St. Joseph County, Ind.) 

The greater part of the northern half of North America 
is covered with a sheet of mantle rock similar in essential 
character to the ground moraine now forming under the 
glaciers of the Alps, Alaska, and Greenland. In the 
United States this sheet of mantle rock extends as far 
south as the Ohio and Missouri rivers. Its thickness 
varies from a few feet to several hundred feet, its average 
depth being not less than ioo feet. The greater part of 
its mass consists of a stony clay containing pebbles of all 
sizes, many of which are glaciated. There are also exten- 
sive deposits of sand and gravel, often well assorted, but 
also mixed with each other and with clay in all proportions. 
More conspicuous than these, but constituting only a small 



THE DRIFT SHEET OF NORTH AMERICA 123 



percentage of the whole, are thousands of boulders of all 
sizes up to that of a small house. The pebbles and 
boulders represent a great variety of material. An hour's 
search is often sufficient to collect fifty or one hundred 
species of rock nearly all foreign to the region where they 
lie, and the major- 
ity of them foreign 
to the United States. 
Most of these " errat- 
ics " or " lost rocks " are 
recognizable as frag- 
ments of the igneous and 
metamorphic rocks of 
the old Laurentian high- 
land of Canada, and in 
some instances they can 
be traced back to a defi- 
nite locality from which 
they must have come 
originally. Large masses 
of metallic copper from 
the shores of Lake Supe- 
rior have been found 
buried in the soil of In- 
diana, and some of them 
are glaciated. Boulders 
of a peculiar conglomer- 
ate, consisting of pebbles 
of red jasper dissemi- 




Fig. 89. — A boulder. 
(Near Camden, Maine.) 



nated through a ground mass of white quartz, are scattered over Ohio, 
Indiana, and Illinois, and on account of their striking colors attract 
popular attention. They must all have come from one parent ledge of 
similar rock on the north shore of Lake Huron. 

The surface of this sheet of mantle rock is traversed by 
a complex system of ridges which have the form and com- 
position peculiar to terminal moraines. In hundreds of 
places where the bed rock has been exposed by natural or 



124 THE LAND 

artificial means it is found to be glaciated, the grooves and 
scratches having a general north-south direction. 

These features admit of but one explanation. This 
sheet is a vast deposit of glacial drift. The evidence has 
now accumulated in such mass, variety, and accordance 
as to make it impossible to doubt that at a comparatively 
recent period the northern part of North America was 
covered with an ice sheet like that of Greenland, extend- 
ing as far south as the glacial boundary shown on p. 125. 

If the lines of glacial scratches are traced back northward, they point 
to the region around Hudson Bay as the location of the central snow 
field. From this region the ice moved southeastward over New Eng- 
land, southward over the Middle states, south westward over the West- 
ern states, westward nearly to the Rocky Mountains, and northward 
toward Alaska and the Arctic Ocean. The farthest point from the 
center reached by the ice was in Kansas, a distance of 1500 miles. The 
area covered was about four million square miles, but it is not probable 
that it was all covered at any one time. 

The Older Drift. — Close examination reveals the fact 
that the drift is not simple and uniform over the whole area, 
but is made up of several distinct sheets which overlap 
one another, like the shingles on a roof. The lowest and 
outermost sheets together form what is known as the older 
drift, which lies on the surface in parts of Ohio, Indiana, 
Illinois, Iowa, Missouri, Kansas, and Nebraska. 

The margin of the older drift is not usually marked by a ridge or 
terminal moraine, but thins out to a vanishing edge along the glacial 
boundary from central Ohio westward. The older drift is extended 
and partly covered by deposits of fine silt (loess), probably the allu- 
vial sediment deft by glacial floods. It is also characterized by the 
occurrence within its mass of buried timber and vegetable debris, so 
common that the well diggers call such an accumulation "Noah's brush- 
heap " or "Noah's barnyard." It is probable that the ice sheets which 
deposited the older drift advanced to their southernmost limit and at 
once retreated without pausing anywhere in the United States long 
enough to form a well-marked terminal moraine. 




!25 



126 THE LAND 

The Newer Drift. — Partly overlapping the older drift 
lies a much thicker and more complex sheet of more recent 
drift, the southern margin of which is marked by a series 
of. terminal moraines, almost continuous from Cape Cod to 
Alberta. 

As will be seen from the map. p. 125, the terminal moraine in the east 
coincides with the glacial boundary, but in central Ohio the two part 
company. In the Mississippi valley they are 500 miles apart, but run 
close together again through the Dakotas. In the interval between 
them in Wisconsin there is a large area entirely free from drift. The 
drift sheet is quite thin in New England, but increases in mass west- 
ward until -in Ohio and Indiana it attains a depth of from 100 to 500 feet. 

The Moraines. — The principal irregularities of the sur- 
face of the newer drift are due to the long lines of terminal 
moraines which traverse it. The margin of the ice sheet 
which deposited the newer drift not only occupied the line 
of its farthest advance long enough to deposit a massive 
moraine, but during its retreat it halted at frequent inter- 
vals or temporarily readvanced. The line held at each 
period of halting is marked by a moraine roughly parallel 
with the previous one. The method of retreat was a step 
backward and then a long pause, as an army retreating 
from an enemy's country marches by day and at night 
halts and throws up intrenchments. 

Between the Ohio River and Lake Superior the lines of moraines 
indicate sixteen successive halting places. A very noticeable feature 
is the looped or festooned form of the moraine groups, indicating that 
the ice sheet was divided into several lobes or tongues which advanced 
independently of one another. This lobation was due to the broad, 
open valleys of the region. In the valleys the ice was thicker than 
on the bordering highlands, and consequently advanced farther and 
melted back more slowly. The basins of the Laurentian lakes seem 
to have exerted a controlling influence upon the lobing of the ice 
margin. There was an Erie lobe in Ohio and Indiana, a Saginaw 
lobe from Lake Huron in Michigan and Indiana, a Lake Michigan lobe 
in Michigan, Indiana,' and Illinois, a Green Bay lobe in Wisconsin, and 



THE DRIFT SHEET OF NORTH AMERICA 127 




Fig. 90. — Map of Erie moraines. 

a Superior lobe in Minnesota. In each lobe the ice spread out from 
the center toward the margin, and in the reentrant angles between the 
lobes piled up iuterlobate moraines of huge proportions. 

The Surface of the Moraines. — At the edge of the ice 
the newer drift material was dumped pell-mell in long 
heaps, while a portion of the ground moraine was pushed 
forward and a portion gathered under the edge where the 
ice current was too feeble to carry it farther. In many 
places the morainic material was deposited in shallow lakes 
which stood along the ice front, or was carried by outflow- 
ing streams far down their valleys. The moraines formed 
under these conditions have a varied aspect. The simplest 
are long ridges or swells, rising above the level surface of 
the drift plain like dead ocean waves. The most massive 
consist of a belt of hills from two to twenty miles wide, 
where the drift is piled in a confused assemblage of 



128 



THE LAND 



domes, knobs, peaks, and irregular ridges, with corre- 
sponding hollows between, all in the utmost disorder. 




Fig. 91.— A hilly moraine. 

(St. Joseph County, Ind ) 



The predominating materials are gravel and sand. The 
feebler moraines present the same features on a smaller 
scale, forming the " mound and sag " type of surface ; or 
the sags may be absent and the moraine consist of a belt of 




Fig. 92. - A kettle hole. 
(Near Morristown, N.J.) 



low, broad mounds rising from a plain. A moraine line 
is sometimes marked only by a broad belt or strip of sur- 



THE DRIFT SHEET OF NORTH AMERICA 



129 



face thickly strewn with large boulders. The relief of a 
morainic surface forms a unique type of topography, which 
once seen and understood can be readily recognized. 

Kettle Holes form one of the most characteristic features of terminal 
They are bowl-shaped or funnel-shaped basins of all sizes 



moraines. 




Fig- 93- — A kame. 

(Tippecanoe County, Ind.) 

and depths, having no outlet, and often occupied by small lakes. Each 
marks the place where a large block of ice detached from the main mass 
and partly buried in drift has melted and left a depression, as ice melt- 
ing under sawdust often does. 

Kames are heaps of'sand and gravel which have been deposited along 
or near the edge of the ice by outflowing streams of water. They take 
the form of mounds and wind- 
ing ridges with a hummocky 
and rapidly undulating outline. 
The material is more or less 
perfectly stratified. They oc- 
cur in connection with mo- 
raines and are often difficult 
to distinguish from them. 

Eskers, or " serpent kames, 11 
are long, winding ridges of 
gravel which ex«tend often for 
many miles across hills and 
valleys in the direction of ice 
movement. They are accumulations formed in the tunnels of sub- 
glacial streams or in ice-walled canyons open to the sky. 

Drumlins are peculiar rounded and elongated lenticular hills of boul- 
der clay, which were formed under the ice some distance back from the 




Fig. 94. — An esker. 

(In Auburndale, near Boston, Mass.) 



130 



THE LAND 




Fig- 95- — A drumlin. 

(Near Amherst, Mass.) 

margin, and perhaps correspond to the sand bars in a river. They do 
not usually occur singly, but in groups which occupy the whole face of 
the country. 

General Results of the Ice Invasion. — The foreign 
boulders arid glacial scratches upon the White, Green, 
and Adirondack mountains indicate that the ice overrode 

their summits and 



::: 



was not less than a 
mile thick over 
northern New Eng- 
land. Its thickness 
over the Laurentian 
highlands may have 
been two miles. 

On account of the ab- 
sence of land projecting 
above it, the surface of 
'the ice sheet was clean, 
and lateral and medial 
moraines were wanting. The drift was gathered up from the bottom, 
and, except a portion which in some manner became incorporated in 




Fig. 96. — Glaciated rock. 
(Summit of Mt. Monadnock, N.H.) 



THE DRIFT SHEET OF NORTH AMERICA 131 

the body of the glacier, was dragged along as a ground moraine. The 
ice sheet may be pictured as combining the features of the Greenland 
ice cap with those of the Malaspina Glacier. The great central expanse 
was smooth and clean, but for many miles back from its margin it was 
probably covered with gravel and boulders laid bare by the ablation of 
the upper layers. It may even have resembled the Malaspina in sup- 
porting a growing forest. 

The action of the ice sheet was vigorous and prolonged 
and its effects correspondingly great. The present sur- 
face features of the region which it covered are largely 
the result of its work. The regions of ice accumulation 
in Canada and New England were regions of greatest 




Fig. 97. — Drift plain. 

(Tippecanoe County, Ind.) 

abrasion. Not only were they swept nearly bare of 
mantle rock, but hills and mountains were worn down, and 
the surface of the bed rock was pitted with thousands of 
shallow depressions now occupied by lakes. The rock 
debris thus formed was carried southward and spread 
over southern Canada and northern United States. In 
the region of glacial deposition, previously existing hills 
and ridges were rubbed down, valleys were filled up, and 
the surface of the country plastered over with a coat of 
drift, as a mason plasters a rough stone wall with mortar. 



132 



THE LAND 



Except in the mountain regions the old surface features 
were obliterated and a new and much smoother surface 
was created. The contrast between the broken surface of 
the country south of the glacial boundary (Fig. 98) and 
the monotonous smoothness of the drift plain north of it 
(Fig. 97) is very striking, and in some places the change 
from one to the other is abrupt. 




Fig. 98. — Dnglaciated region. 

(Near New Albany, Ind.) 

The Drift Plain is relieved only by shallow valleys which the streams 
have cut a little way into it and by the belts of morainic hills which rise 
here and there from fifty to three hundred feet above its surface. The 
moraine belts are studded with thousands of ponds and small lakes, and 
the plain itself abounds in swamps and marshes. On account of the 
gentle slopes and the short time during which the streams have been 
at work, the whole region is poorly drained. The drift, however, is the 
" grist of the glacial mill,'" and consists of an intimate mixture of rock 
flour and fragments ground from a great variety of minerals. It contains 
all the elements of plant food and forms one of the most productive 
and enduring soils in the world. The drift regions are preeminent in 
their agricultural resources. 

The Preglacial Drainage of the glaciated region underwent profound 
modification. The St. Lawrence River system was completely changed 



THE DRIFT SHEET OF NORTH AMERICA 



133 



in character, a subject which will be more fully discussed in the next 
chapter. The old outlet of the Allegheny and Monongahela rivers, 
which formed a single northward-flowing stream, was dammed with drift, 
and their waters were permanently diverted to the Ohio. The northern 
outlet of the Winnipeg basin in Canada was dammed and the basin 
occupied by a fresh-water sea (see Lake Agassiz on map, p. 125), which 
emptied through the Minnesota River into the Mississippi. The course 
of the Missouri through the Dakotas was displaced one hundred miles 
to the westward. From the Maine coast the ice extended far out to 
sea, the lower portions of the stream valleys in Maine were deepened, 
and by subsequent drowning they have been converted into fiords, 
which give the coast its present extremely ragged outline. 




Fig. 99. — Glaciated regions of Europe. 

Other Glaciated Regions. — During the glacial period 
northern Europe passed through a series of ice invasions 
similar to those of North America, and it now presents 
similar characteristic features. The general movement of 



134 THE LAND 

the ice, the glacial boundary, and the principal terminal 
moraine are shown upon the map, Fig. 99. The Scandina- 
vian mountains formed the chief gathering grounds, with 
secondary centers of dispersal in the Scotch highlands 
and the Alps. The Baltic and North seas were filled with 
solid ice, as Hudson Bay was. 

The glacial period closed at least 10,000 years ago, yet 
it was so recent as compared with other great changes, 
and its effects in Europe and North America were so pro- 
found and far reaching, that it may well be regarded as one 
of the most important events in the recent physical history 
of the world. 

Realistic Exercises. — Any student who lives north of the glacial 
boundary should make himself acquainted with the glacial features in 
his vicinity. Boulder clay may be readily found and distinguished 
from other clay ; foreign pebbles and boulders may be picked up by 
the thousand. Glaciated pebbles and glacial scratches on the bed 
rock are likely to be found anywhere. Deposits of loess, the thickness 
of the drift, the occurrence of buried timber, drift-filled valleys, changes 
in stream courses, moraines, kettle holes, lakes, kames, eskers, and^ 
drumlins should be looked for and investigated. No region offers 
better facilities or more interesting subjects for elementary field work 
than the areas covered by glacial drift. 



CHAPTER XL 
LAKES AND LAKE BASINS 

In an ideal drainage basin the slope is continuous from 
the divide to the mouth of the stream. But in nature slopes 
are interrupted by depressions which are completely sur- 
rounded by a rim of higher land and act as reservoirs which 
detain and store up a portion of the rainfall. Such a de- 
pression, if the rainfall is 
sufficient, fills with water 
up to the level of the 
lowest point in the rim 
and becomes the bed of 
a lake or pOnd. Lakes 
may be regarded as ex- 
pansions of the streams 
with which they are con- 
nected. They vary in 
form and size from quiet 
pools or reaches, where 
the current of a stream 
is imperceptible, to veri- 
table inland seas, like the 
Great Lakes. 

Diastrophic Basins 

The Great Basin.— The 

largest basins are due to Fi e I0 °- 

the warping or irregular elevation and depression of the 
earth-crust by internal forces. The Great Basin of west- 

i35 




136 THE LAND 

ern United States, lying between the Sierra Nevada and 
Wasatch Mountains, has an area of about 210,000 square 
miles. Its surface is divided by parallel mountain ranges 
into numerous valleys and subordinate basins. The rain- 
fall is scanty and almost confined to the mountain tops. 
Great Salt Lake in Utah is the shrunken remnant of a 
body of water (Lake Bonneville) which was nearly ten 
times as large as the present lake, stood about 1000 feet 
higher, and had an outlet by way of the Snake River to the 
Columbia. During the period of overflow its waters were 
fresh, but a decrease in rainfall caused its surface to fall 
below the level of the outlet, and it has become increas- 
ingly salt. At various levels around its inclosing rim, its 
former shore lines, with their wave-cut cliffs, bars, spits, 
terraces, and deltas, record the work of the waves and in- 
flowing streams of the ancient lake (see Fig. 132). 

Lakes of Nevada. — At the time of the greatest extension of Lake 
Bonneville a large body of water (Lake Lahontan) occupied a very 
irregular area of 1500 square miles in the western side of the Great 
Basin, receiving drainage from the Sierra Nevada. This lake, at its 
highest stage, had a depth of nearly 900 feet, but never had an outlet. 
Pyramid, Winnemucca, Walker, Humboldt, and Carson lakes now 
occupy the lower portions of the old lake bed. They are subject to 
great variation in volume from year to year. At many points in the 
Great Basin, wet weather lakes gather in times of rainfall and soon dry 
away, leaving ftlayas, or beds of mud, which bake to a hard and cracked 
crust. 

Asiatic Basins. — The great plateaus of Asia include ex- 
tensive basins and inland drainage systems similar to those 
of the Great Basin of the United States. The country lying 
between the Kuenlun and Altai Mountains is of this char- 
acter. The desert of Gobi is the bed of a dried-up sea, 
containing in its lowest parts salt lakes and marshes which 
rise and fall with the uncertain water supply. Snow-fed 



LAKES AND LAKE BASINS 



137 



streams creeping out from the mountain valleys sometimes 
reach these lakes, but their waters are generally lost or 
evaporated. Southwestern Asia, including large portions 
of Persia and Arabia, contains basins either entirely dry or 
holding in their lowest parts shrunken salt or bitter lakes. 

The Caspian Basin. — To the north and west of the pla- 
teau country of Asia, and including a large area of south- 
eastern Europe, lies a great low 
plain which has no outlet to the 
sea. It contains the Caspian Sea, 
which has an area of 1 70,000 square 
miles and a maximum depth of 3000 
feet. The surface of the Caspian 
Sea is about 90 feet below the level of 
the sea. The Caspian seems to have 
been originally part of a gulf which 
extended southward from the Arctic 
Ocean, from which it was cut off by 
the rising of the intervening land. 
The Aral Sea and Lake Balkash 
are salt lakes of the same origin as 
the Caspian. 

Rift Basins. — In east Africa there are 
two extensive chains of lakes and dry basins 
which are long and narrow and lie, like 
fiords, between precipitous cliffs thousands 
of feet in height. One chain extends from 
Lake Nyassa on the south, through Tan- 
ganyika and Albert, to Rudolf, where it is joined by another chain from 
the south. Thence the line continues northward as a long strip of low 
land, dotted with lakes and old lake basins, some of which are below 
sea level, to the southern end of the Red Sea. At the north end of the 
Red Sea a similar line of depressions extends from the Gulf of Akabah 
to the Dead Sea and the valley of the Jordan River in Palestine. This 
deep, narrow valley, nearly 4000 miles long, containing the Red Sea and 

DR. PHYS. GEOG. — 9 




Fig. 101. — Map of east Afri- 
can lake chains. 



138 THE LAND 

more than thirty lakes, has been produced by a series of parallel faults, or 
cracks in the earth-crust. The block between the faults has subsided, 
forming the " Great Rift Valley," bounded by high, precipitous walls 
on either side. The bottom is quite irregular and has been obstructed 
by many outflows of lava. Lake Nyassa is 350 miles long, 50 miles 
wide, and 300 to 600 feet deep, and empties southward by the Shire 
and Zambezi rivers to the Indian Ocean. The largest lake, Tangan- 
yika, is 400 miles long, 20 to 40 miles wide, and 500 to 2000 feet deep, 
and overflows westward into the Kongo River and Atlantic Ocean. 
Lake Albert is one of the sources of the Nile. Most of the lakes have 
no outlet. The Dead Sea is remarkable for the extreme saltness of its 
waters and for the fact that its surface lies nearly 1300 feet below the 
level of the sea. 

In the northwestern part of the Great Basin there are numerous rift 
valleys, some of which are occupied by small lakes. Of these, Alvord 
and Warner valleys in Oregon, Surprise Valley in California, and Long 
Valley in Nevada are the most notable. Long and Warner valleys 



STEIN MTS. 




Fig. 102. — Section of Alvord Valley, Oregon. 

are continuous, and form a narrow basin 100 miles in length, walled in 
by sheer precipices in some places 2000 feet high. The Stein Moun- 
tains rise 4000 to 5000 feet above Alvord Lake. (See Figs. 150 and 
151.) A series of rift valleys extends from central New Mexico, through 
western Texas, into Mexico. 

Glaciated Basins 

Lakes are more numerous in glaciated regions than in 
any other parts of the world. (See maps of northern 
Europe and North America.) Basins in glaciated regions 
are of two classes: (1) bed-rock basins, most numerous in 
regions where the ice was thickest and abrasion most active ; 
and (2) drift basins, most numerous in regions of glacial 
melting and deposition of drift. The great moraine sys- 
tems which stretch across the United States from Cape 



LAKES AND LAKE BASINS 



139 



Cod to the Dakotas, and across Europe from the Valdai 
Hills to Denmark, are belts of small lakes. Morainic 
basins due to the irregular deposition of drift are ex- 
tremely variable in form, but may be classified as kettle, 
channel, and irregular basins. 



MORAINIC LAKES 

NORTHEASTERN INDIANA. 
Moraines 5 
Beaches 




Fig. 103. 

Kettle basins or kettle holes (Fig. 92) are roundish, caldron-, or funnel- 
shaped depressions which owe their existence to the melting of detached 
masses of ice left, during the glacial retreat, partly buried in drift. 
Those which have a clay bottom are filled with water, but those with a 
gravel bottom are generally dry. Channel basins are long and narrow, 
and were made by streams which temporarily drained the melting ice 
front. Irregular basins are combinations of kettle holes, channels, and 
other depressions which fill and overflow into one another, forming 
connected bodies of water at the same level. 



140 



THE LAND 




Fig 104. 



The Finger Lakes. — Glaciated regions abound in long, 
narrow rock basins occupied by lakes, among which those 



LAKES AND LAKE BASINS 



141 



in central New York called, on account of their form and rela- 
tive positions, the Finger Lakes, are of peculiar interest. 

The northern slope of the Alleghany plateau is here trenched by 

many long, narrow valleys from 1000 to 2500 feet deep, some of which 

contain lakes, while many do not. Of larger 

and smaller lakes there are more than a 

^"Vgc dozen. Seneca and Cayuga —e each 




Fig. 105. — Profile of northward slope of Finger Lake plateau. 

about forty miles long and one to three miles wide. Seneca is 441 feet 
above sea level and 618 feet deep. Cayuga has an elevation of 378 feet 
and a depth of 435 feet. 
On the plateau between the 
lakes tributary streams flow 
in broad, shallow valleys 
until within a short distance 
of the lake, where the val- 
leys end in the air, as if cut 
off (Fig. 106), and the 
streams drop into deep, nar- 
row gorges whjch continue 
almost to the lake shore. 
Small deltas and alluvial 
terraces occur at various 
elevations on the hillsides. 
The Finger Lakes oc- 
cupy basins which were in 
preglacial times the valleys 
of streams flowing into the 
Ontario basin. When the 
ice sheet moved from 
the north over this region it 
was comparatively thin on 
the ridges, but much thicker 
in the valleys. By glacial abrasion the valleys were widened and deep- 
ened, and the slopes on either side made more steep. During the 




Fig. 106.— Taughannock Falls, near Cayuga Lake. 



142 THE LAND 

retreat of the ice the ridges were probably uncovered first, while con- 
siderable masses of ice still occupied the valleys. As the ice gradually 
melted and the lake surfaces fell from one level to another, the tributary 
streams on the plateau entered the lakes at different levels, forming 
deltas and terraces which they afterward cut through or abandoned, 
building others at lower levels. The lake basins had been so much 
deepened by glacial erosion that the old tributary valleys were left far 
above trit present lake levels, and the streams, compelled to cascade 
down the steep slopes, began to cut back their present gorges or glens. 



SCALE OF FEET 



1000 2000 3000 



Fig. 107. — Cross section of Cayuga Lake valley. 

(Vertical and horizontal scales the same.) 

The Cayuga basin was probably made 350 to 450 feet deeper by ice 
abrasion ; but the valley is still only a broad, shallow groove. 

Mountain Valley Basins. — The Alps and other moun- 
tain regions which have been recently glaciated contain 
many basins similar in essential characteristics to those of 
the Finger Lakes. The valleys head in vast cirques or am- 
phitheaters upon the flanks of the mountains, the sites of 
former neve fields, whence they descend by irregular steps 
through successive basins to the lowest, which lies near 
the end of the old glacier and sometimes extends out into 
the surrounding plain. The lower ends of these basins 
are usually bordered by morainic dams. The Italian lakes, 
Como, Lugano, Garda, and Maggiore, and the Swiss lakes, 
Geneva, Constance, Zurich, and Lucerne, occupy basins of 
this kind. Landslips, moraines, and deposits of sediment 
by lateral streams have built the natural dams which hold 
back the waters of many mountain lakes, but their basins 
are due largely to glacial erosion. Such mountain lakes 
are numerous in Scotland, Scandinavia, New Zealand, and 
the Rocky Mountains. 



LAKES AND LAKE BASINS 



143 



The Scotch highlands have been subjected to very extensive glacial 
erosion, and contain numerous lakes, of which Loch Katrine is one 
of the most beautiful. Its 
length is eight miles, its 
width one mile,and its depth 
495 feet. It fills a single 
symmetrical basin which is 
closed at its lower end by a 
belt of very hard and dura- 
ble rocks. During the later 
stages of the glacial period 
the direction of ice move- 
ment was down the valley, 
which was scooped out to a Fig Io8 ' 

depth of 130 feet below sea level; but the more resistant rocks were 
left as a barrier which holds the water up to its present level. 

The Laurentian Lakes. — Of special importance, on 
account of their great size and interesting history, are the 
Great Lakes of the St. Lawrence system. Lake Superior 
is the largest body of fresh water on the globe, and it con- 
tains 280 of the 570 cubic miles of water stored in this 
chain of reservoirs. 

Their areas, levels, and depths are given in the following table : — 




Loch Katrine. 





Area 


Elevation, ft. 


Maximum 


Average 




in sq. mi. 


above sea level. 


depth in feet. 


depth in feet. 


Superior 


31,200 


602 


1,008 


475 


Huron 


23,800 


581 


73° 


250 


Michigan .... 


22,450 


581 


870 


325 


Erie 


9,960 


573 


2IO 


70 


Ontario 


7,240 


247 


738 


300 



They occupy a series of comparatively elongated basins, 
separated by small areas of land, and joined near their 
ends by short streams or straits. The shape of the lakes, 
their nearness to one another, their end connections, 



144 



THE LAND 



and their trend suggest an overgrown stream line with a 
succession of immense reaches, a repetition on a grand 
scale of the characteristics of almost any meadow brook. 
They occupy basins which seem to be the broken and 
obstructed sections of a great stream valley. This impres- 
sion is strengthened by the course of the line of greatest 
depth, as shown by the heavy line in Fig. 109. The bot- 
toms of all the lakes except Erie are below sea level. 



MAP OF THE PREQLACIAL 
LAURENTIAN RIVER 

S — Superior Valley 
P—Pewamo Valley 
T- Trent Valley 
D—Dundas Valley 
C— Cuyahoga Valley 




Fig 109 

Evidences of Glaciation. — The whole St. Lawrence basin was 
deeply buried under the ice sheet, evidences of which are abundant 
in the grooved and scored rock surfaces on the islands and shores of 
the lakes, in the lobed and irregular shape of their southern shores, 
and in the arrangement of the terminal moraines of the glacial lobes 
around and between them (see map, p. 125). 

Evidences of Tilting. — The lake basins are surrounded by numerous 
old beaches or shore lines, which mark the height and limits which 
their waters have at some time reached. But these old shore lines are 
no longer level. They gradually rise toward the north and east. One 
of them, known as the Algonquin beach, is 25 feet above the southern 



LAKES AND LAKE BASINS 145 

end of Lake Huron, and 635 feet above its northern end. The depth 
of the lakes below sea level, and the extensive drowning of the St. 
Lawrence valley (see p. 95), show that the whole basin onee stood at 
a considerably higher level than at present ; while the occurrence of 
bones of the whale and other marine animals along the shores of Lake 
Ontario and Lake Champlain shows that these lakes and the lower 
St. Lawrence valley once formed a great arm of the sea, an extension 
of the Gulf of St. Lawrence. All these facts point to the conclusion 
that the basin of the St. Lawrence has been subjected in the past to 
extensive depression and upheaval, which was in the nature of a tilting 
along a northeast and southwest line. 

Many of the peculiarities of the basins of the Great 
Lakes may be attributed to the agency of the Laurentide 
ice sheet, which, creeping forward from the Canadian high- 
lands, flowed into, filled, and crossed these basins. The pre- 
glacial valley of the Lauren tian river was probably widened 
and deepened in some parts by the removal of material, 
and obstructed in other places by its deposition. Borings 
have revealed many deep valleys which lead into or con- 
nect the lakes, but are now filled with drift. Among these 
are the Pewamo or Grand River valley across Michigan, 
the Trent valley between Georgian Bay and Lake Ontario, 
the Dundas valley between Erie and Ontario, and the 
valley of the Cuyahoga at Cleveland (see Fig. 109). The 
slight depth of Lake Erie indicates that its basin was not 
a part of the main valley, but a tributary to it. 

With the exception of Lake Superior, which is an old 
diastrophic basin, the Great Lakes are old river valleys, 
first cut wide and deep by weathering and stream erosion, 
then depressed, uplifted, and tilted by movements of the 
earth-crust, and finally widened, deepened, and cleaned 
out here and choked up and obstructed there, by the 
North American ice sheet. 

Ice-dammed Lakes. — When the ice sheet began to melt away, and 
the southern divide of the Laurentian basin was uncovered, the water 



146 



THE LAND 




{'J- LAURE N-T ID p , , — MI !- ES , 

J 6 .*,? . <,. .. ,-;<• .. V u E /,q ^ So Too 




collected at several points 
along the ice front and formed 
a number of temporary lakes 
of varied and changing size 
and form. They were bounded 
and held in on the north by 
the wall of the retreating ice 
front, but their outlines and 
outlets can still be traced by 
the beaches formed where 
their waves beat against the 
land. Figure no shows three 
out of the many successive 
stages in the long and compli- 
cated history of the Laurentian 
lakes. 

Lake Agassiz. — The largest 
of the ice-dammed lakes of 
the period of glacial recession 
occupied the basin of Red 
River in Minnesota and North 
Dakota and extended far north- 
Fig, no. 




LAKES AND LAKE BASINS 



147 



ward into Canada (see map, p. 125). Its outlet was through the Minne- 
sota River into the Mississippi, but the opening of an outlet through 
the Nelson River into Hudson Bay drained its waters until only Lakes 
Winnipeg and Winnipegosis remain. Its sediments now form the soil 
of the great wheat fields of the Red River region. 



Barrier Basins 

. Many examples have already been cited of basins which 
are partly due to the formation of natural dams or bar- 




Fig, in. —A barrier basin. 
(Lake McDonald, in the Rocky Mountains, Mont.) 

riers across a valley. There are few lakes which do 
not owe their existence, more or less, to this cause. The 
dam may be of hard rock, as in Loch Katrine ; of glacial 
drift, as in the Great Lakes ; or a terminal moraine, 
as in the case of mountain lakes. A lava stream from 



148 



THE LAND 




Fig. 112. — Intermorainic lakes, Idaho. 



a volcano sometimes obstructs a valley and forms a 
coulee lake. Landslides often form temporary dams, be- 
hind which water accumulates for a time and then breaks 
through with destructive violence. The deposit of a tribu- 
tary stream may 
set back the waters 
of the main trunk 
into which it flows. 
The growth of coral 
reefs and the forma- 
tion of sand bars re- 
sult in the cutting 
off of portions of a 
sea or lake from the 
main body of water. 
Thus shore lagoons of great variety and extent are pro- 
duced. Probably glacial moraines act more often as 
barriers to drainage than any other species of dam. 
Wherever they occur in series, the valleys between usually 
contain many intermorainic lakes. 

Other Basins 

Volcanic Basins. — Crater Lake, in southern Oregon, is 
circular in form, with a diameter of five miles and a depth 
of 2000 feet. Its surface is 6239 feet above sea level, and 
it is bordered all around by precipitous cliffs from 500 to 
2200 feet high. From the crest of the encircling rim the 
country slopes away on all sides. The roughly bedded 
layers of rock also slope outward and downward from the 
lake shores. The lake, as its name suggests, occupies 
the crater of an extinct volcano. The angle of its 
slopes indicates that its summit may have been a mile 
above the present lake surface. The material which once 



LAKES AND LAKE BASINS 



149 




,--"""' 


Crni 


er Lake ~"~~fe^rr | 


Sea Level 



Fig. 113. —Crater Lake, Oregon. 

formed the cone and filled the crater was probably not 
blown out by an explosion, but has disappeared by sinking 
into the depths from ,.- x 

which it came. Basins 
of this kind are not nu- 
merous, but Lakes Alba- 
no and Averno in Italy, 
the Laacher See in Ger- Fi «- »4.- crater Lake, Oregon, 

many, and Lake Taupo in New Zealand belong to this class. 

Alluvial Basins. — The oxbow or horseshoe lakes, very common in 
flood plain regions, result from the cutting off of a river bend and the 
silting up of its ends. ' They have been fully discussed in connection 
with the Mississippi River. 

Basins by Solution. — In regions of limestone rocks, where subterra- 
nean drainage channels exist, the falling in of the roof of a cavern 
often forms a sinkhole basin which partly fills with water. Deep pits or 
wells are sometimes formed by the escape of water through beds of 
salt, gypsum, or other soluble rock. This is the origin of many of the 
small lakes in Florida. 



150 THE LAND 

The Relation of Lakes to Rainfall and Drainage. — The 

existence of a lake in any basin depends upon the amount 
of rainfall, which must exceed the amount of water re- 
moved by percolation and evaporation. In arid regions 
the rainfall is generally insufficient to fill the basins to 
overflowing, the minerals brought in by tributary streams 
accumulate, and the water becomes salt or alkaline. Such 
lakes may dry up or fill with mineral deposits, leaving 
thick beds of salt, soda, borax, gypsum, or tufa ; but on 
account of the absence of outlet streams which would cut 
down the rim, such basins are relatively permanent. In 
regions of abundant rainfall, lakes regulate the flow of out- 
let streams, preventing floods. They also act as settling 
basins for sediment, so that a stream flowing out of a lake 
is usually clear. 

Of all features of the landscape, lakes are the most 
ephemeral. A combination of agencies is at work toward 
their speedy destruction. The inflowing streams are fill- 
ing them with sediment and minerals deposited from 
solution, while the outflowing streams are cutting deeper 
channels through the retaining ' barriers. In the case 
of small, shallow lakes the growth of vegetation is one 
of the most efficient agents of destruction. Aquatic 
plants find anchorage and rich soil in the lake bottom, 
while they absorb the greater bulk of their food from the 
atmosphere. They grow and decay year after year, and 
the lake becomes filled with vegetable matter. Thus it is 
gradually converted into a peat bog or a muck meadow. A 
lake may be buried by the accumulation of vegetable matter 
which floats upon its surface. A large majority of lakes 
occur in glaciated plains, or in rugged mountain regions ; 
that is, upon land surfaces which have not been long ex- 
posed to atmospheric agencies. They are characteristic of 



LAKES AND LAKE BASINS 



151 



the youthful stages in the development of relief. The 
lakes of arid regions are no exception to this rule, because 
erosion and deposition go on there with extreme slowness. 

Realistic Exercises. — The student should make a list of all the agents 
and processes concerned in the formation of lake basins and classify 
the basins according to their origin. The topics to be considered 
in summary and review are: the most efficient agents in basin forma- 
tion, the relation of lakes to climate, to rivers, and to the development 
of relief, and the general absence of lakes from old mountain regions 
like the southern Appalachians, and from extensive plains like southern 
Russia and southeastern United States. 

The formation of basins and some of the characteristics of lakes may 
be studied in any locality, at least on a small scale. Any lake, pond, 
or pool exhibits some of the phenomena and processes peculiar to the 
life history of a lake, just as a small brook has many of the characteristics 
of a large river. Let the student investigate the origin of the basin, 
whether it be by excavation or damming or both. Inflowing streams 
are filling the basin with sediment and, perhaps, building deltas at 
their mouths. The outlet 
stream is usually clear, 
but may be cutting its 
channel deeper and 
lowering the water level. 
This lowering may be 
hastened by an artificial 
ditch. Low, level, and 
marshy land about the 
borders of the lake shows 
the former extent of the 
basin and the amount of Fi § "5- ~ Shore lines on S ravel bank 

filling which has taken place. Old beaches or shore lines may be found 
at some distance from the water's edge. The growth of vegetation 
may be noted, and the formation of peat around the shores. A tem- 
porary pool or puddle of water formed during wet weather and disap- 
pearing in a few days often furnishes an opportunity for the study of 
shore lines at successively lower levels, either cut into the face of a little 
cliff or built up where the shore is shelving. When it dries up, the 
fine mud brought in by feeding streams is left as a coating on the bot- 
tom and hardens into a genuine ftlaya. 




CHAPTER XII 
THE DEVELOPMENT OF DRAINAGE SYSTEMS 

The Life History of a River. — By the action of running 
water the face of the land is being carved into ever-chan- 
ging patterns of relief. While this work is going on, the 
streams themselves undergo a parallel series of changes. 
Every stream system has its life history, during which it 
develops from a stage of youthfulness, when it has just 
begun the task before it, through maturity toward old age, 
when its possible work upon the land has been accom- 
plished. From a study of existing streams we may picture 
to ourselves an ideal river which passes through this 
series of changes without accident or interruption. 

For the simplest case, suppose a considerable area of 
the earth-crust to be slowly elevated above the sea. Let 
it be composed of rock strata originally in a nearly hori- 
zontal position ; but suppose the strata, while in the process 
of elevation, to be somewhat crumpled and folded, forming 
a long ridge from which the surface slopes toward the sea 
in opposite directions. The result of this movement is 
a coastal plain, rising gradually into a plateau and then 
more steeply to a dividing ridge. The surface is slightly 
irregular, diversified by broad, shallow basins or elongated 
depressions, with broad, flat divides. 

This newborn land is exposed to a temperate climate 
and a moderate rainfall. Weather and running water be- 
gin their work at once. 

The run-off is at first in sheets rather than in streams. 
Each basin is filled until the water runs over into the next 

J 52 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 153 

lower one, and each long depression transmits a shallow 
flood until continuous lines of waterway are established 
from the high land to the sea. A waterway whose course 
is determined by the original irregularities of the surface 
of its basin, is called a consequent stream. The waters 
charged with sediment begin to corrade the surface over 
which they flow, and soon engrave it with delicate channel 
lines. The valleys contain numerous lakes, but extend in 
the general direction of the steepest slopes to the sea. 
Each stream has its steepest slope near the divide, but its 
volume there is small because it drains a small area. Near 
the sea it has a large volume but a gentle slope. There- 
fore the middle portion, having the requisite volume and 
swiftness, intrenches itself most rapidly. 

At first each drainage line is an almost limbless trunk, 
but as it sinks its channel deeper into the earth-crust, 
the lakes are drained and lateral branches appear which 
traverse the intervals between the main streams. As 
the main stream deepens its channel its branches are 
given a steeper slope, their currents are quickened, and 
they develop other branches as the main stream has done. 

Thus the drainage system grows by extending its 
branches upward and outward like a tree, until their 
tips reach the crest of the main ridge and interlock with 
the tips of the branches of the next system on either side. 
Water falling upon any portion of this land finds a system 
of continuous channels by which it returns to the sea, and 
the drainage is complete. 

In the middle portion of a river, where corrasion is most 
rapid, a larger number of strata are cut through, some of 
which prove to be harder and some softer : consequently 
rapids and cataracts appear which retreat upstream, leav- 
ing gorges below them. On account of the disturbed and 

DR. PHYS. GEOG. — IO 



154 THE LAND 

crumpled condition of the rock strata in the. highest ridge, 
alternations of hard and soft layers are frequent, and the 
head waters present a series of cascades which persist a 
long time because the streams are too small to wear the 
strata away. 

In the lower reaches of a river the valley is soon cut 
down to base level, where the slope is gentle and the cur- 
rent too slow to carry the full load of sediment it receives. 
Deposition occurs and downward corrasion ceases. The 
stream begins to swing from side to side, to undermine its 
bluffs, and thus to widen its valley. Floods are frequent, 
and spread over the wide valley floor their successive layers 
of sand and mud. Thus a wide flood plain is built up and 
becomes characteristic of this part of the river's course. 

Meanwhile the river may be depositing a delta at its mouth and 
pushing it out to sea, and thus building a clam of sediment. The base- 
leveled and flood-plain condition gradually extends itself up the main 
stream and thence up the larger tributaries in succession, until at length 
corrasion and valley deepening continue only in the torrential head 
waters and in the middle portion, where it is less vigorous than at 
first. 

The head-water streams are small in volume, and the load they carry 
is the coarsest, but they are able to move it because of their steep 
slopes. In the middle course the load of sediment gathered by the 
tributary streams is large, but in its descent from the upper valleys it 
has been ground finer. The volume of water is larger and its flow 
is sufficiently swift to enable it to carry its greater load. In the lower 
course the load is still larger and finer, but the volume of water is pro- 
portionately great, and through all its writhings and shiftings the river 
staggers on, dropping sediment here and now, and picking it up again 
there and hereafter, but in the long run getting it delivered finally to 
the sea. 

A river which has acquired a perfect adjustment be- 
tween its volume, slope, and load is said to be a mature, 
graded stream. It is a condition toward which all streams 
are tending, but which few ever reach. By their degree 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 



155 



of approach to it, not by years, is their age reckoned. A 
man at twenty years is young, but a horse of twenty is old, 
not in time, but in stage of development. 

If a stream system should ever reach base level through- 
out the extent of its trunk and principal branches, and 
should reduce its basin to a plain faintly sloping from low, 
indefinite divides to wide flood plains, it would have reached 
a condition of old age. 

The Development of Valleys. — By downward corrasion 
a stream cuts a steep-sided trench of its own width, but 
weathering, gravitation, and the wash of the rain widen it 
and make the sides sloping. The form of the valley in 
cross section varies with the stage of development, and 
with the material into which it is cut. A very young val- 
ley is a simple V-shaped d o__jb a b c a 

groove, as may be seen 

in any hillside gully. As 

time goes on, it becomes 

deeper and more flaring, 

as shown in b and c, Fig. 116. When base level is reached, 

and the valley passes into a flood-plain condition, the form 

is radically changed, as in d. If the 

walls of the valley contain strata of 




BASE LEVEL 

Fig. 116. 



c b a b c 




unequal hardness, the 
form is modified by 
the projection of hard 
layers and the retreat 
of the softer ones, as 
117. If the strata are 



Fig. 117. 

shown in Fig. 

not horizontal, various unsymmetrical 

forms are produced, as in Fig. 118. 

The Development of Divides and Profiles. — The divides 
and ridges between stream valleys pass through a corre- 




i 5 6 



THE LAND 




sponding series of changes. They are at first broad and 
flat or gently rounded, as a— a, Fig. 119. As the valleys 
widen, the interstream h c ch , 7 

d c b a r-~7fi -\-;t— - « b c d_ 

ridges grow narrower and 

sharper and are finally 

lowered. A slope made ba s £ level 

irregular by weathering is Flg - " 9 - 

steeper in hard material and more gentle in soft, and the 
tendency of water running over it is to wear away the pro- 
cb a jecting corners, where the flow is swiftest, more 
rapidly than the reentrant angles, where the flow 
is slowest. The tendency also is to leave 
only the coarse sediment near the top of the 
slope and to spread the finer far 
Fl «- I2 °- out from its foot (Fig. 120). It 

follows from these conditions that slopes produced by 
weathering tend to be irregular, that a perfectly graded 
slope grows steeper toward the top, and that its pro- 
file is a curve concave upward, with the 
greatest curvature at the upper 
end. This is called the curve 
of corrasion or stream 





Fig. 121. 



Curves of corrasion in a stream and its eWSWtl (Fig. I2l). 
tributaries. 



A graded slope is usually flattened at the top, where the rivulets 
run only while it rains and are too feeble to corrade. The curve of 
ram-wash is convex upward. The relative ex- 
tent of these two curves depends mainly upon 
elevation. On high plateaus and mountains the 
curves are concave to the very top and the ridges 
are sharp and angular (ABC, Fig. 122). On 
hills and plains the curves are convex, and the 
elevations are broad and rounded (BD, Fig. 122). Combinations of 
the two curves exist in all proportions. The progress of erosioii tends 
toward the final flattening of all slopes. 




Fig. 122. 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 157 



The Migration of Divides. — During the development 
of drainage systems a struggle goes on between adjacent 
trunk streams for possession of the territory. Some rivers, 
by reason of larger volume, steeper slope, or softer mate- 
rials to work in, are able to extend their branches and 
head waters more rapidly than others, and thus to push 
back the divides and invade the basins of their neighbors. 
This may occur gradually by a general widening of the 
valley of the stronger stream, as shown in Fig. 119, or it 
may occur rather suddenly by capture or piracy. 




Fig. 123. 

The Chattooga River, at the western corner of South Carolina, was 
formerly the upper part of the Chattahoochee ; but the Savannah had 
a shorter course to the sea and a more rapid fall. One of its tributaries 
was able to extend itself until it tapped the Chattahoochee and robbed 
it of its head waters LM (Fig. 123). The divide was thus shifted from 
the line AB to the line AC. The Oconee will probably repeat this 
process in the near future. An elbow or right angle in the general 
course of a river is frequently an indication that it has thus beheaded 
one of its neighbors, as at E. 



i 5 8 



THE LAND 



The St. Joseph River in southern Michigan was originally the upper 
part of the Kankakee in Indiana. While the edge of the Michigan 

ice lobe stood along the termi- 
nal moraine, a large stream 
now represented by the Dowa- 
giac drained the ice front and 
emptied into the Kankakee at 
the site of South Bend. When 
the ice withdrew, the Dowa- 
giac turned aside through a gap 
in the moraine to the basin 
of Lake Michigan. A portion 
of its channel was thus aban- 
doned by the main stream 
and left to transmit in a re- 
versed direction a small tribu- 
tary. This tributary had a fall 
of more than three feet to the 
mile, while the Kankakee had 




Fig. 124. 



only one third as much ; and it did not take very long for the more 
rapid stream to eat back the low divide at its head, and to divert the 
St. Joseph from the Kankakee and Mississippi to Lake Michigan and 
the St. Lawrence. 

The Development of Meanders. — A straight stream is 
an impossibility in nature. The current through an arti- 
ficial ditch soon shows a tendency to become crooked. A 
slight inequality in the firmness of the bank, or an acci- 
dental obstruction, is sufficient to turn the current to one 
side, from which it is deflected toward the other, and 
incipient meanders are established. 

The course of a stream consequent upon the irregulari- 
ties of a surface newly raised above the sea or renewed by 
glacial action is crooked in an irregular manner. As a 
stream continues its work it develops meanders accord- 
ing to its conditions. The steeper the slope and the swifter 
the current, the more direct is the course. As the stream 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 



159 



approaches base level the available energy is so reduced 
that a slight obstruction is sufficient to turn it aside, and 
it develops in its flood plain the wide curves so character- 
istic of large rivers (see maps of the Mississippi). These 
are roughly symmetrical because the material of the flood 
plain is nearly homogeneous. 

Subsequent Streams. — As consequent streams deepen 
their channels they are liable to find differences in the 
hardness of the rock over which they flow, and are 




A. Consequent drainage. 




B. Subsequent drainage, with water and 
wind gaps. 
Fig. 125. 



obliged to adjust themselves to these conditions. Suppose 
the young consequent streams shown in Fig. 125 A to 
flow down a moderate slope across which two strata of 
hard rock extend at right angles to the streams. The 
strongest stream (b) is able to cut gaps through the hard 
strata more rapidly than the weaker ones. It extends its 
branches to the right and left in the softer strata, and 
finally not only beheads its neighbors, but dismembers 
them, and adds their fragments to its own system, as 



160 THE LAND 

shown in Fig. 125 B. A stream which has thus adjusted 
its system to the structure of its basin is called subsequent. 

The map of such a system looks like a grapevine trained upon a 
trellis, and it is hence called trellised drainage (Fig. 125 B). Where 
the main stream crosses the softer strata, weathering and lateral corra- 
sion are most rapid, and the valley is wide and open ; but its level can 
not be sunk lower than that of the stream where it cuts through the 
hard stratum below. Each hard stratum acts as a dam which deter- 
mines the base level of the stream above. In this dam the river slowly 
cuts a narrow notch, which, in the progress of erosion, becomes a sluice 
or gateway through a ridge standing out above the level of the more 
easily eroded country on either side. Such a gateway is called a water 
gap (see Fig. 159). The shallower notches made by the other streams 
before they were dismembered, and now abandoned, remain as wind 
gaps. 

Disturbances of Stream Development. — It is very sel- 
dom, if ever, that a stream is permitted to pass through 
all the stages of development from youth to old age in 
a regular and normal manner. The most important inter- 
ruptions arise from elevation or subsidence of the stream 
basin and from glaciation. The general effect of elevation 
is to put new life and vigor into a stream by giving it a 
steeper slope. It begins over again the work of deepen- 
ing its valley and extending its head waters. 

Thus a new set of narrower, deeper, and straighter waterways are cut 
down into the floors of the old ones, and rocky shelves or terraces are 
left to mark the old valley floors (see Figs. 51, 52). Subsidence has an 
effect the reverse of that due to elevation and makes a stream prema- 
turely old. Its slopes are diminished, its current slackens, the lower 
portion of its valley is drowned, the middle portion fills up with sedi- 
ment, and only the head waters are able to continue actively the work 
of corrasion. Changes of climate which increase or diminish the rain- 
fall have a corresponding effect upon the volume and force of streams. 

Glaciation. — Some of the effects of glaciation upon 
streams have already been noticed. During the existence 
of an ice sheet nearly all the streams in the glaciated 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 161 



region are temporarily obliterated, and after its disap- 
pearance their courses are found to be permanently di- 
verted, or their valleys dammed and choked with drift. 





Fig. 127. — Cross section of a filled valley. 
(T, terrace; R, river; F, flood plain.) 



Fig. 126. — Alluvial terraces. 
(Mississippi valley, near St. Cloud, Minn.) 

Many of the valleys which drained the ice front were flooded with 
water and half filled with sand and gravel. Since the disappearance of 
the ice the diminished streams 
have been at work cleaning out 
their old valleys, with imperfect 
success. In most cases, the «o- 
stream has cut a new and 
smaller channel through the 
drift filling, leaving massive 
alluvial terraces on either side. 

Summary. — A young stream is one which has accom- 
plished but little of the work of erosion and degradation 
of the land which is possible for it to do. It is charac- 
terized by irregular profile, steep slope, swift current, 
narrow valley, and numerous rapids, cataracts, and some- 
times lakes. The Colorado River is an extraordinary ex- 
ample of a young river. The St. Lawrence, once a 
mature stream, has been rejuvenated, or restored to infancy, 
by glaciation. 



162 THE LAND 

A mature stream is one which is well advanced in the 
work before it. It has graded or nearly graded its valley, 
and its profile is nearly the ideal curve of corrasion (see 
Fig. 121 ). Its valley is broad and its lower portion is in 
the flood-plain condition. The rapids, cataracts, and lakes 
have disappeared or linger only along the head waters. 
The Mississippi river system has nearly reached the stage 
of x maturity. 

An old stream is one which has reduced its basin to the 
lowest possible level. It would resemble the lower Mis- 
sissippi ; but there is probably not a river in the world 
which has reached that stage throughout. Rivers are sel- 
dom permitted to reach old age, but are either drowned by 
the sea or restored to youth by the accidents of upheaval 
or glaciation. 

A consequent stream is one whose course is determined 
by the relief of the surface of its basin. All streams are 
at first consequent. 

A subsequent or adjusted stream is one whose course 
has been modified by the structure of its basin. The 
relief of its basin is largely the result of its own work. 

An antecedent stream is one which has maintained its 
original consequent course in spite of upheavals of the 
land and the growth of plateaus and mountain ranges in 
its basin. Some portions at least of the Green-Colorado 
system are antecedent. 

A superimposed stream is one whose course has been 
determined by the relief of some previously existing sur- 
face, which has been removed or greatly modified by ero- 
sion. Its course, like that of an antecedent stream, has 
little or no relation to the present relief. When the con- 
sequent streams of the glacial drift have swept it away 
and laid bare the surface of the bed rock beneath, they 



THE DEVELOPMENT OF DRAINAGE SYSTEMS 163 

will become superimposed. Some portions of the Green- 
Colorado may have been superimposed from a surface 
which has since been removed by degradation. The 
Susquehanna, Delaware, and Potomac are superimposed 
streams (see pp. 185, 186). 

Streams as Factors in Human Life. — Drainage. — The 
natural function of streams is drainage. They carry away 
the surplus water supplied by rainfall, in excess of that 
which is evaporated. Most of the ground-water finally 
reaches the streams. Drainage is most rapid and complete 
where the slopes are steep, the surface compact, and vege- 
tation scanty. In mountainous regions generally these 
conditions prevail in a high degree, the rainfall runs off 
quickly, and the streams are subject to great variations in 
volume, being flooded during a storm, and nearly dry in 
clear weather. The removal of forests and the cultivation 
of the ground render drainage more rapid and complete. 
More rapid run-off carries away greater loads of mantle 
rock, and hastens the process of stream erosion. In some 
cases, the destruction of forests has resulted in washing 
away the soil and leaving the surface barren and worth- 
less. On the other hand, slow and imperfect drainage 
may be as objectionable as too rapid drainage. A region 
which is imperfectly drained abounds in marshes, ponds, 
and lakes, which support a vegetable and animal life pe- 
culiar to themselves. Such regions are comparatively 
unfavorable for human occupation until their drainage is 
artificially improved. The cutting of ditches and the 
laying of underdrains may remove the stagnant water, 
and render the rich accumulations of humus, or decayed 
vegetable matter, available for the growth of agricultural 
plants. Some of the best areas for farming and gardening 
have been obtained in this way. 



1 64 THE LAND 

Flood plains. — The most productive lands in the world 
are flood plains. At every period of high water, a stream 
brings down mantle rock from the higher grounds, and 
deposits it as a layer of fine sediment over its flood plain. 
A soil thus frequently enriched and renewed is literally 
inexhaustible. In a rough, hilly, or mountainous country, 
the finest farms and the densest population are found on 
the "bottom lands" along the streams. The flood plain 
most famous in history is that of the river Nile in Egypt. 
For a distance of 1500 miles above its mouth this river 
flows through a rainless desert, and has no tributary. The 
heavy spring rains which fall upon the highlands about 
its sources produce in summer a rise of the water, which 
overflows the valley on either side. Thus the lower Nile 
valley became one of the earliest centers of civilization, 
and has supported a dense population for 7000 years. 
The conditions in Mesopotamia, along the Tigris and 
Euphrates rivers, are similar to those along the lower 
Nile, and in ancient times this region was the seat of a 
civilization perhaps older than that of Egypt. The flood 
plains of the Ganges in India, and the Hoang in China, 
are the most extensive in the world, and in modern 
times the most populous. The alluvial valley of the Mis- 
sissippi is extremely productive of corn, cotton, and sugar 
cane. 

Irrigation. — In all ages the extent and value of flood 
plains have been increased by artificial means. Dikes 
or levees are built to regulate the spread and flow of the 
water and to protect the land from destructive floods. 
Dams and reservoirs are constructed for the storage of 
water, which is led by a system of canals and ditches to 
irrigate large tracts of land which would be otherwise 
worthless. By means of irrigation, the farmer has control 



STREAMS AS FACTORS IN HUMAN LIFE 165 

of his water supply and is able to get larger returns than 
are possible where he depends upon the irregular and 
uncertain rainfall. It is estimated that in the arid regions 
of western United States there are 150,000 square miles 
of land which may be made available for agriculture by 
irrigation. Perhaps in the future the valley of the lower 
Colorado may become as productive as that of the Nile. 

Routes of travel and commerce. — Streams are the easiest 
routes of travel and commerce. In uncivilized countries 
they are almost the only ones. Explorers take advan- 
tage of them to penetrate the interior from the sea, and 
the first settlers follow the same routes. A river usually 
furnishes from its mouth well up toward its source a 
smooth, graded highway, upon which a cargo may be 
transported with much less effort than overland. If 
obstructions occur in the form of rapids or falls, boat 
and cargo are carried around them. It is often easy to 
pass by a short portage or " carry " from one stream 
system across the divide to another. In a new country, 
homesteads, towns, and cities spring up first along the 
streams, and the density of population and the value of 
land decrease back toward the divides. As the country 
becomes more thickly settled, wagon roads and railroads 
are built, and the waterways become less important. But 
in regions which are not very level the easiest grades in 
every direction are found along the streams, and the main 
routes of land travel follow the stream valleys. In 
traversing a mountainous region, a railroad follows the 
windings of some river up to the crest of the divide, which 
it crosses through a pass, or often by a tunnel, and 
descends the valley of some stream on the other side. 

If a stream is too shallow or too much obstructed 
for easy navigation, a canal may be built either around 



166 THE LAND 

the obstructions or through the whole length of the 
valley. In the most highly developed countries the 
large rivers still form highways of commerce. Their 
mouths furnish good harbors for sea-going vessels, and 
lighter craft can penetrate far inland. The Mississippi is 
navigable to St. Paul, iooo miles from the sea, and the 
St. Lawrence with the Great Lakes carries more tons of 
freight than any other inland route in the world. 

Water power, etc. — Streams with rapid fall furnish water 
power available for running machinery. Regions where 
such streams are numerous and accessible, as in New 
England, furnish unusual facilities for manufacture, and 
the country becomes densely populated. A fall or rapid 
upon .any stream is likely to become the site of a manu- 
facturing and commercial center. The water power at 
the Falls of Niagara, now partly utilized, is sufficient to 
run all the machinery in several of the largest cities on 
the continent. 

Thus, by furnishing easy routes for transportation, and 
cheap power for manufacturing, rivers have determined 
the principal lines of human travel and migration, and the 
principal centers of human settlement. Nearly all the 
large cities of the world, and thousands of smaller ones, 
owe their location and growth to the advantages furnished 
by some stream. 

Streams furnish a supply of fish and other animals val- 
uable for food or clothing. Uncivilized peoples often 
depend upon them mainly for support, and in cases like 
that of the salmon of the Columbia and other streams of 
the Pacific slope, fish become an important article of com- 
merce. Streams have trenched their valleys into the crust 
of the earth and exposed the bed rock, which is thus made 
accessible to the quarryman. 



STREAMS AS FACTORS IN HUMAN LIFE 167 

Sources of knowledge. — The long and often deep sec- 
tions cut by streams enable us to observe the structure of 
the earth-crust, and to read its history. In this respect no 
other river has done so much for us as the Colorado. In its 
canyons the layers of stratified rock are exposed down to 
the granite core of the earth, and we have been able to 
interpret there the story of the past, and to learn the 
processes by which the various forms and features of the 
land have been produced. 

Beauty of scenery. — Man is largely indebted to streams 
for the variety and beauty of scenery. Running water 
itself is attractive to young and old. A landscape without 
water lacks its chief charm. A child instinctively finds its 
way to the brook, and the man seeks beside the river 
the pleasure and recreation which no other place affords. 
Streams have carved the surface of the land into an end- 
less variety of beautiful forms, and a land where stream 
valleys are few or shallow is monotonous and tiresome. 
The most common as well as the most celebrated beauty 
of scenery in the world, from the tiny meanders of a 
meadow brook to the unequaled grandeur of the Colorado 
canyons, is largely due to the presence and action of 
streams. 



CHAPTER XIII 
FORMS OF SEDIMENTATION 

Run-off of Mantle Rock. — There is not only a constant 
run-off of water from the face of the land, but also a run- 
off of the land itself, and the two are closely connected. 
A snow field accumulates by additions at the top until an 
ice sheet is formed which escapes down a valley or spreads 
over a large part of a continent. A sheet of mantle rock 

grows by progressively 
deeper weathering of 
the bed rock at the bot- 
tom. If it rests upon 
a nearly level surface it 
may remain where it 
was formed and consti- 
tute a residual soil ; but 
if formed upon a slope, 
it creeps downward. 
On very steep slopes 
enormous masses of earth sometimes slip suddenly, form- 
ing a landslide analogous to an avalanche of snow. 

Gravity is the chief agent, but it is assisted by the forces of freezing 
and thawing. The movement is most rapid along the face of a vertical 
cliff, from which fragments fall by their own weight and form a talus 
with a slope as steep as the character of the material will permit. 

Wind Deposits. — The wind is an important factor in 
the movement of mantle rock, transferring it up grade 
as well as down. Along the shores of the sea and of 
large lakes, the winds from the water blow the sand up 




Fig. 128. —A landslide. 

(Sawtooth Mountains, Ida.) 



FORMS OF SEDIMENTATION 



169 




Fig. 129. — Talus slopes. 
(Devils Lake, Rocky Mountains.) 

in wavelike heaps which resemble the sand ripples in 

bottom of a stream, sweeping it up the long slope 

the crest and dropping 

it over the other side. 

Thus the dime ( Fig. 131), 

as it is called, travels 

slowly inland and buries 

forests and farms which 

may lie in its way. 

The most extensive dune 
systems are found in desert 
regions, where the sand is con- 
tinually passing through a se- 
ries of shifting forms like the 
waves of the sea. The loess 



the 

to 




Fig. 130. —Sand ripples. 
(Algerian Sahara.) 



which occurs so plentifully in connection with the glacial drift 
p. 124) is thought by some to be, at least in part, a wind deposit. 

DR. PHYS. GEOG. — 1 1 



(see 



170 



THE LAND 



Glacial Drift. — The agency of ice in the transportation and removal 
of mantle rock has already been discussed in detail (see Chapters IX 
and X). Drift sheets and plains, moraines and drumlins, are forms 
assumed by mantle rock on its way to the sea when deposited by ice. 




Fig. 131. —Sand dune on the shore of Lake Michigan. 

Kames and eskers are similar forms which are produced by the com- 
bined action of ice and water. When a glacier reaches the sea a portion 
of its load is floated away by the icebergs and distributed over the sea 
bottom as they melt. Ten pounds of ice is sufficient to float one pound 
of rock. 

Alluvial Deposits. — The movement of mantle rock would 
be very sluggish indeed if it were not for the fact that 
with the help of running water the rock assumes a con- 
dition in which it is virtually liquid. Suspended in water 
the sediment is transported according to the laws explained 
in Chapter IV. 

A slight decrease in the velocity of a stream greatly 
diminishes its power to carry sediment. The coarsest 
part of its load is dropped first, and afterwards the finer 
parts. Running water thus has a remarkable power of 
assorting materials. If its velocity is gradually and con- 
tinuously checked there is a continuous deposit of sedi- 



FORMS OF SEDLMENTATION 



171 



ment, varying downstream from coarser to finer. If its 
velocity is suddenly checked, coarse and fine sediments 
are deposited together without much assorting. As the 
volume and velocity of a stream vary at any given place, 
it may deposit there at different times coarse or fine sedi- 
ment or none at all. Thus alluvial deposits are always 
characterized by stratification, or division into more or less 
distinct layers. Each layer is made up of similar frag- 
ments, and represents a period of continuous deposition. 
Each division plane represents a pause in deposition, or 
an abrupt change in the character of the material. 

Alluvial Cones and Fans. — A mountain stream with a very rapid fall 
brings down a mass of coarse debris, and at, the foot of the slope, where 




^"/f^a^ 



'm# 



■e 



Fig. 132.— Alluvial cone. 
(Near Salt Lake City, Utah.) 

the current is suddenly checked, deposits it in the form of a steep 
alluvial cone. Where the sediment is finer it is spread out into a low, 
broad alluvial fan. 

Alluvial Plains. — The alluvial plains formed in the 
lower courses of great rivers have already been described 



172 THE LAND 

(see pp. 74-78). In some cases a diastrophic valley may be- 
come filled with sediment to a great depth and be converted 
into an alluvial plain. One of the most extensive alluvial 
plains in the world is the valley of California, between the 
Sierra Nevada and the Coast Ranges. It is 400 miles long 
by 80 miles in width, and has been filled with sediment, 
brought mostly by the streams from the Sierra, to the depth 
of two thousand feet. 

Lake basins which have been filled with sediment form 
lacustrine plains. 

Deltas. — A delta differs from an alluvial fan in being a 
deposit in still water instead of on land. It is generally 
larger, broader, and flatter than a fan, but the two are not 
always easily distinguished. Any stream, large or small, 
flowing into a pond, lake, or sea, may build a delta 
if the conditions are favorable. Strong waves, tides, and 
currents in the receiving body are unfavorable conditions, 
but large rivers are able to accomplish the work in spite 
of them. 

One of the largest deltas in the world, that of the Ganges-Brahma- 
putra, has been built in the Bay of Bengal where the tides rise and fall 
sixteen feet. Large deltas are but extensions of flood plains, and they 
grow more rapidly where the water off the mouth of the river is shallow. 
Their extension is often retarded by the fact that the crust of the earth 
slowly sinks under the load of sediment, as seems to be the case at the 
mouth of the Mississippi, where the deposit has acquired a vertical 
thickness of a thousand feet or more. 

Coast Shelves. — Through whatever forms mantle rock 
may pass on its downward way, its final destination is the 
bottom of the sea. The streams of ice and water dis- 
charge it into the shallows along the shore, and the waves, 
tides, and currents help to distribute it over a. wider belt. 
The coarser material is not carried far beyond the exist- 
ing shore line, but the very finest may be lodged several 



FORMS OF SEDIMENTATION 



173 



hundred miles out. Thus along the coast of every land 
mass the coast shelf is being built up at a rate which 
varies with the quantity of sediment and the depth of the 
water. 

Deposits from Solution. — Another portion of the material 
carried from the land remains to be noticed. It is not 
mantle rock and it plays no part in the construction of the 
forms just described. Those mineral constituents of the 
earth-crust which are dissolved by rain water become 
actually liquid and their run-off is free and rapid. In the 
Mississippi River the quantity of mineral matter carried 
in solution forms more than one fourth of the whole 
amount discharged into the Gulf. It consists chiefly of 
carbonate and sulphate of lime and common salt. 




Fig 133. — Alkali plains, Arizona 

Ground-water is nearly everywhere charged with lime, salt, and other 
minerals in varying quantities, and Wherever it evaporates from the 
surface of the earth a saline crust is slowly formed. Thus in dry 
regions the soil gradually becomes charged with " alkali,"' forming 
"alkali plains." 

In lakes which have no outlet streams vast quantities of salts accu- 
mulate. Beds of rock salt hundreds of feet thick are of frequent occur- 



174 



THE LAND 



rence in the earth-crust, and mark the sites of ancient lakes or seas 
which have dried up. Every -stream delivers to the sea its quota of salts 
in solution, and sea water would grow indefinitely more salty if it were 
not for the agencies which bring about a partial deposit of its mineral 
matter. 

Foremost among these agencies is animal life. Most of the animals 
and some plants which live in the sea have a shell or bony skeleton 
composed of lime which the animal or plant extracts from the water. 
When the organism dies the skeleton sinks to the bottom, and con- 
tributes to the formation of a lime deposit. Animals and plants thus 

convert the dissolved lime into an 
organic sediment which is subject 
to the same laws as any other sedi- 
ment. One third of the sea bottom 
is covered with a soft gray ooze or 
mud made up entirely of the shells 
of minute animals which live in the 
surface waters. This deposit only 
needs consolidation to produce rocks 
resembling the chalk that is abun- 
dant in England and other parts of 
the world (see p. 33). 



Coral Reefs. — The most 
peculiar and interesting accu- 
Fig 134. -ooze, magnified. mulations of limestone in the 

sea are the coral reefs. The rock of coral reefs is chiefly 
made up of the skeletons of various species of the coral 
polyp. The individual polyp varies in size from a pinhead 
to a foot or more in diameter, but most of them are small. 
In the reef-building species the individuals are not free 
and separate, but thousands of them are connected to- 
gether in one head or mass which is attached to the bot- 
tom and grows by branching out somewhat like a bush. 
The young polyp is produced by budding from the side 
of the parent, and remains attached to the parent stem. 
The combined group of individuals secretes a lime skeleton 




FORMS OF SEDIMENTATION 



175 




which forms a foundation upon which new generations 
build, a head of coral being alive only at the tips of the 
branches. These animals flourish in warm, clear water 
where a strong cur- 
rent brings them 
plenty of food. They 
can not live in 
muddy water, or if 
exposed to the air, 
or at depths greater 
than about 300 feet. 

The bottom of the sea 

is traversed by numerous 

ridges from which vol- 

, . , Fig. 135. — Branching coral, 

canic peaks rise nearly 

to the surface or project above it, forming small islands. The submerged 
peaks present conditions favorable to the growth and multiplication of 

myriads of sea animals, 
whose remains accumu- 
late until a shoal or bank 
covered with shallow 
water is formed. Upon 
this foundation patches 
of coral begin to grow 
upward and to spread out as far as the shallow water extends. As the 
top of the patch approaches the surface of the sea (Fig. 136, a), the polyps 
near the center die from „ _ , 

Sea Level 

crowding, want of food, 
occasional exposure to 
the air, and the deposit 
of mud and sand, while 
those near the edges, having plenty of room, food, and clear water, con- 
tinue to grow and extend outward (Fig. 136, b). The waves break off 
pieces of the living coral rock and pile them up on the reef until its 
edge is raised in some places five to fifteen feet above the sea. At the 
same time the water dissolves out the dead coral rock from the middle 
(Fig. 136, c). The result is an irregular, broken ring of land surround- 




Fig. 136. 



■■■■■^■■iii 

Fig. 137. —Section of actual coral reef. 



176 



THE LAND 



ing a shallow lagoon or lake of water. These ring-shaped islands, or 
atolls, are of all sizes, from one to a hundred miles in length, but the 




Fig. 138. — An atoll. 

area above water is very small, often consisting of a few islets rising 

here and there from the submerged reef. 

A volcanic island often has a fringing reef of 
coral along its shore. In other cases it is sur- 
rounded by a barrier reef some distance out from 
the shore, where the food supply and the absence of 
mud favor a vigorous growth of coral. The waves 
are continually breaking up the reef and grinding it 
into calcareous sand and mud, which fill the spaces 
between the coral branches. The whole mass rap- 
idly becomes cemented into solid limestone rock 

which shows little or no indication of its origin. Thus the tops or 

slopes of the volcanic peaks become crusted over with limestone known 

in one case to be at least 1000 feet thick. 




Fig- !39- — Map of an 
atoll. 




Fig. 140. — Part of a coral reef. (Great Barrier Reef, Australia.) 
(From a photograph loaned by the American Museum of Natural History, New York.) 



FORMS OF SEDIMENTATION 177 

Coral reefs and islands are most numerous in the west- 
ern Pacific Ocean. They also occur in the northern part of 
the Indian Ocean and in the western Atlantic. All the 
extensive coral formations lie in the track of the great 
equatorial currents (see p. 265), which bring to the growing 
polyps abundance of warm water and food. They are ac- 
cumulations of lime which has been extracted from solution 
in sea water, solidified, and deposited by living plants and 
animals. Their place in the economy of nature corre- 
sponds to that of the salt beds and alkali plains on land. 

Results of Sedimentation. — By these various processes 
of sedimentation the material washed away from the land 
is finally deposited on the bottom of the sea, being assorted 
and laid down in layers which are level or only slightly 
inclined. Each layer of mud or sand is somewhat irregu- 
lar in thickness and limited in extent, gradually thinning 
out to an edge where it overlaps and is overlapped by 
other layers. By pressure and the cementing action of 
sea water, probably assisted in the deeply buried layers by 
the internal heat of the earth, the beds of mud and sand 
near the shore are gradually consolidated into strata of 
shale and sandstone, while farther from shore, beyond. the 
reach of the coarser sediment from the land, limestones are 
formed by the rain of shells upon the bottom. Thus mantle 
rock is reconverted into bed rock, the forms destroyed 
upon land are reconstructed in the sea, and over thousands 
of square miles the thickness of the earth-crust is increased 
by the addition of sedimentary rocks in horizontal strata. 

Realistic Exercise. — The student should return to the study of the 
streams in his vicinity in the light of Chapters XII and XIII. Nearly all 
forms of slopes, valleys, divides, cones, fans, alluvial plains, terraces, 
and deltas may be found ; or they may be made at will by the use 
of a mound of earth and a sprinkling pot. 



CHAPTER XIV 

MOUNTAINS 







w®jm~-\~ -^szWm^s&k 



fig. 141. — Folded strata, Maryland. 

Faulted and Folded Strata. — As shown in the last 
chapter, sedimentary rocks are always laid down in nearly 
horizontal strata. The greater part of the land surface is 
found to be made up of (or underlain by) similar strata which 
must have been formed originally under similar conditions, 
and subsequently upheaved to their present elevation (see 
Figs. 14, 15, 18). Over large areas of plains and plateaus 
the strata have been lifted bodily upward with little or no 
displacement of parts or disturbance of their original 
smooth horizontality. In other regions they have been 
tilted, folded, and broken into every degree of confusion. 

A fault is a fracture accompanied by displacement of the strata. 
It may be accompanied by a bending up or down of the strata. The 

178 



MOUNTAINS 



179 



amount of throw or vertical ' 
displacement is sometimes 
as much as 20,000 feet. 

A syncline is a downfold- 
ing of the strata in the form 
of a trough, as at a, Fig. 143. 

An anticline is an up- 
folding of the strata in the 
b 





Fig. 142. — A fault. 

form of an arch, as at b, Fig. 143. 
An anticline is sometimes over- 
thrust, as in Figs. 144 and 145. 

Compressed folds are a series 
ofsharply bent synclines and anti- 
clines in which the connecting: 




Fig. i44- Fig. 145. 

limbs are parallel and nearly vertical, as shown in Fig. 146. 

A fan fold is an anticline which has been pinched at the bottom 
until it is narrower there than at the top, as shown in Fig. 147. 

In nature these forms are seldom ,,- .., 

found complete, but more or less ' / \ 

extensively eroded. 




i8o 



THE LAND 



Mountains. — Any relatively great elevations of land 
having steep slopes and a sharp or narrow top are popu- 
larly regarded as 
mountains. The term 
includes features 
which vary widely 
in form, dimensions, 
structure, and origin. 
A single mountain 
rarely exists by itself 
except in the case of 
volcanic cones (see 
Chapter XV). A 
mountain range is a 
long ridge having 
two principal slopes 
and a crest, which 
may be jagged or 
level like the ridge- 
pole of a barn. It 
may be straight and 
simple, or, as is more commonly the case, it may be curved 
or irregular and send out many branching spurs. 

The important part of the range is not the peaks, which may attract 
attention, but the broad continuous mass which supports them (Fig. 
149). Usually but a small portion of a range can be seen at once or 




iP^r ^ 



L4 



Fig. 148. 



An irregular mountain range. 
(Park Range, Colo.) 




Fig. 149. — Teton Range, Wyoming. 



MOUNTAINS 



ISI 



shown in a picture ; hence erroneous ideas are apt to be acquired con- 
cerning their steepness, ruggedness, and confusion. 

Ranges are often found combined in chains, and chains 
in systems. Mountains are usually portions of the earth- 
crust which have been not only uplifted but also deformed 
and dislocated. The special character of the mountains 
is primarily determined by the nature of the deformation. 

Block Mountains. — Probably the simplest mountains in 
existence are those of southern Oregon and northern Cali- 
fornia and Nevada. This region is traversed by numerous 
ridges which extend north and south and are separated 
by barren valleys and playa lakes. They are from 10 to 




Fig. 150. — Section of block mountains. 

100 miles long, 3 to 20 miles wide, and 1000 to 5000 feet 
high. They have a steep slope on one side, often rising 
in a sheer precipice to a height of 2000 feet or more, and 
may be easily recognized as broken blocks of the earth- 
crust which have been tilted, some one way and some an- 
other. They resemble 
the roofs of old-fash- 
ioned houses with un- 
equal slopes. The 
long back slopes B, 
Fig. 151, are formed 
by the surfaces of the 
strata, while the steep slopes E are formed by their broken 
edges. T is a trough block, forming a " rift valley " by its 
subsidence. 

Many of the mountain ranges of the Great Basin are of similar 
structure ; but most of them are much more complicated. The younger 




Fig- 151- 



182 



THE LAND 




Fig. 152. —Half-buried mountains, Utah. 



ranges still retain their 
straight crests and even 
slopes. The older ranges 
have been carved by pro- 
longed erosion into ex- 
tremely rugged forms. 
The crests are jagged, 
the slopes are ridged by 
spurs and valleys, and the 
original form has been 
changed until it is hardly 
recognizable. The val- 
leys between are broad 
plains of sand and gravel 
washed from the moun- 
tains. On account of the 
scanty rainfall there are 
few permanent streams to 
carry away the debris, 
which has accumulated 
until the ranges are half 
buried in their own waste. 



Simply Folded Mountains. — The Uinta Mountains in 
northeastern Utah extend east and west about 120 miles. 
The width of the range is forty miles. The crest follows 
an irregular line north of the center, is cut by a few 
shallow notches or passes, and rises at some points to a. 
mile and a quarter above the surrounding plateau. The 
slopes are broken by parallel spurs, which branch out from 
the crest and are separated by transverse valleys. The 
range has been formed out of a single broad arch or flat 
anticlinal fold, which has been greatly eroded. 

The structure is slightly complicated by the occurrence of a fault 
along the north side. From the dip of the strata upon each side, the 
original height of the arch above the present crest has been calculated, 
and it appears that a thickness of three and one half miles of rock has 
been removed. This does not imply that the mountains were ever 



MOUNTAINS 



183 



three and one half miles higher than at present : for erosion has gone 
on at the same time with upheaval. Figure 153 shows in the foreground 



In the background the Uinta 
/old is supposed to have re- 
mained tmeroded, 
the foreground shows 
the Uinta Monn 
ains as they exist 




Fig 153 

the mountains as they are, and in the background the mountains as 
they would be if the eroded material were restored. 

The Jura Mountains in France and Switzerland consist 
of a series of parallel ridges and valleys in which each 
ridge is an anticline and each valley is a syncline. They 
are so young that 
only the topmost lay- 
ers have been eroded 
from the arches, and 
the floors of the 
troughs have been 
thinly covered with 
mantle rock. Fi £- x 54- — Sterogram of Jura Mountains. 

Complexly Folded Mountains. — Few mountain systems 
are as simple as those just described. In most cases they 
owe their existence primarily to extensive foldings and 




THE LAND 

faulting which combine to give them a very com- 
plex structure. Figure 1 5 5 shows the variety of 
structure which exists in the Appalachian high- 
land, and the extent to which erosion has modified 
the original forms in that region. The forms 
produced by the process of upheaval have been 
almost entirely destroyed, and the present moun- 
tains are quite different from the original Appa- 
lachians. Strata having a thickness of at least 
five miles have been removed from the highland, 
and scarcely more than the stumps of the old 
mountains now remain. 

At 1, Fig. 155, a series of compressed folds has been 
worn down to a plain, the surface of which is formed by the 
edges of the nearly vertical strata. At 2 an anticline has been 
reduced to a valley, and at 3 a syncline is left standing as a 
ridge made up of concave strata like a pile of platters. The 
ridges at 4 are projections of hard strata above the more easily 
eroded ones on either side. Most of the present ridges are 
of this character, and their parallel zigzags are shown in Fig. 

156. With level crests rising to a nearly uniform height, they 
stretch across the country like gigantic walls. Adjacent ridges 
frequently approach each other and unite at a sharp angle, 
inclosing a valley, the structure of which is shown in Figs. 

157, 158. In Fig. 157 the bed of hard sandstone which forms 
the ridges is continuous under the valley, and its shape re- 
sembles the prow and bottom of a canoe. The region 
occupied by the Appalachian valleys and ridges is about 
seventy-five miles wide, and extends through Pennsylvania, 
Maryland, and Virginia into Tennessee. It is bounded on- 
the east by a much older mountain range, the Blue Ridge 
(B, Fig. 155), and on the west by the eastern escarpment 
of the Alleghany plateau (A, known as the Alleghany Moun- 
tains), in which the strata have been but slightly disturbed. 

The drainage of the Appalachian highland pre- 
sents many interesting peculiarities. The principal 



MOUNTAINS 



I8 5 




Fig. 156. — Part of the Appalachian Mountains, in Pennsylvania 

rivers, the Delaware, Susquehanna, and Potomac, rise in 
the western plateau and flow southeastward directly across 
the trend of the ridges, through which they pass by means 
of water gaps. The tributary streams follow the valleys 
between the ridges, and 
with their branches 
present systems of trel- 
lised drainage (see p. 
160) in great perfec- 
tion. The main streams 
are independent of the 
relief, and flow across 
the trend of the ridges 
rather than parallel 

with it ^S x 57- — Eroded syncline ; canoe valley. 

Such relations between relief and drainage must be the result of a 
long period of adjustment. The level sandstone-topped ridges of uni- 
form height and the softer strata of shale and limestone in the valleys 

DR. PHYS. GEOG. — 12 




THE LAND 



upon 
were 



between suggest the expla- 
nation. The original Appa- 
lachian folds were in past 
ages worn down to a nearly 
level plain sloping gently to 
the southeast, across which 
the large rivers followed the 
same courses as at present. 
The plain was then uplifted 
to the height of the present 
ridge crests, and as the re- 
vived streams cut their val- 

Fig. 158. —Eroded anticline. 1 • . ■. .1 1 

6 ° leys into it, they came down 

the alternations of hard and soft rocks. While the main streams 

slowly sawing their gaps into the sandstone, the tributaries were 





Fig- 159. —The Delaware water gap. 

able to erode wide valleys out of the limestones and shales (see Figs. 
157, 158, and 160). By a series of adjustments as explained on page 
1 59, all the smaller streams in each valley combined into one system 
tributary to a master stream which was able to cut its gap down more 
rapidly than the rest. Thus the trunk streams became superimposed 



MOUNTAINS 



187 



upon the new surface and their tributaries maturely adjusted to its 
structure. Every ridge and valley in the present Appalachian Moun- 
tains is the product of 
erosion, but the arrange- 
ment of hard and soft 
strata is due to the orig- 
inal upheaval and fold- 
ing. The internal forces 
of the earth furnished a 
block of peculiar struc- 
ture, from which air, rain, 
frost, and running water 
have carved out the pres- 
ent peculiar and elaborate 
pattern of relief. 



I n many lofty moun- 
tain ranges the strata 
have been doubled 
up, crushed, contort- 
ed, overturned, and 
shoved upon one an- 
other in wild con- 
fusion. 




Fig. 160. — Water gaps of the Susquehanna. 



Figure 161 shows a portion of the typical structure of the Alps. The 
central core of such ranges usually consists of granite or some allied 
igneous rock, from which the layers of sedimentary rock which once 
covered it have been removed. The unstratified granite and the 
edges of the highly inclined strata have been split by frost into a thou- 

sand sharp and jagged 

ridges, peaks, needles, and 
"horns." Although the 
peaks, ridges, passes, and 
valleys are conspicuous 
and occupy the whole 
landscape, they are super- 
ficial features, and form but a small part of the great mass of the range 
which supports them (compare Fig. 162 with Fig. 149). 




Fig 161. 



THE LAND 




Alpine scenery. 

Relict Mountains. — In many cases mountains of com- 
plex structure have been worn down to their roots, and 
their surface forms bear apparently little relation to the 
arrangement of the material in their mass. But the peaks 
and ridges mark the place of harder and more resisting 
rocks which have been left prominent by the removal of 
less resisting rocks around them. The mountains of 




Fig. 163. — Mount Monadnock, in southern New Hampshire. 



MOUNTAINS 



189 



southern New England (see Fig. 163) and the Scotch 
Highlands are examples of this class, which may be called 
relict mountains. Their rounded smoothness of outline is 
due to glacial abrasion. 

Plateau Mountains. — Some massive elevations which 
have the appearance of mountains and are popularly so 
called hardly deserve the name. They are really dissected 




Fig 164 —Dissected plateau. 
(Scott County, Tenn.) 

plateaus. The strata have been but slightly if at all dis- 
located by faulting or folding, but are deeply cut by stream 
valleys. The interstream ridges are mountainous in size, 
but the strata of which they are composed remain nearly 
horizontal, and may be traced from one ridge to another 
across the valleys. The Catskill Mountains in New York, 
and the Alleghany plateaus of Pennsylvania and West 
Virginia, are plateau mountains. 



190 THE LAND 

Summary. — The general term mountains includes a 
variety of land forms which differ in their structure and 
origin. They have the common characteristics of large 
mass, elongated outline, and great elevation. They are the 
products of two sets of forces, one of which acts within or 
below the earth-crust to produce elevation, and the other 
acts on the surface of the crust to produce degradation. In 
mountains like the Oregon blocks and the Jura the internal 
forces are supreme and almost wholly responsible for the 
form. In mountains like the Appalachians internal forces 
have folded and dislocated the strata, but the present 
forms are almost wholly due to erosion. Every degree 
of gradation between these extremes may be found. 
In every range internal forces have raised the massive 
block out of which external forces have carved the details 
of ridge, spur, peak, pass, and valley. It follows that 
lofty mountains, like the Himalayas, Alps, Rocky, and 
Sierra Nevada, are lofty because they are young ; and that 
low, subdued mountains of complex structure, like the 
Appalachians and the Scotch Highlands, are low and 
subdued in outline because they are old. 

Earthquakes. — The elevation, depression, folding, and 
faulting of the earth-crust show that it is subject to a 
variety of stresses and strains. When it finally yields to 
an increasing stress and a displacement suddenly occurs, a 
violent jar results, which is propagated through the crust, 
like that which is- painfully felt when a stick bent in the 
hands suddenly breaks. The speed with which the shock 
travels is about three miles per second, and it often extends 
completely through or around the earth. The focus or 
place where the break or slip occurs may be at a depth 
of several miles, but the jar travels upward and out- 
ward in ever-widening circles, diminishing in violence 



MOUNTAINS 



191 



as it proceeds. The surface movements thus produced 
constitute an earthquake, which is most violent at a point 
directly above the focus. The actual distance through 
which any given point of the earth's surface is moved sel- 
dom exceeds a small fraction of an inch, but the velocity of 
the motion may be so great as to make the jar exceedingly 
destructive. Great cracks open in the earth, and from them 
mud and hot water are sometimes expelled. Landslides 
are started, and streams are dammed or turned out of their 
courses. Buildings are cracked or thrown down, and cities 
destroyed with great 
loss of human life. 
Some of the most de- 
structive effects are 
produced by earth- 
quakes under the sea, 
which disturb the water 
so violently that great 
waves rise over the 
shores and sweep every- 
thing before them. Pi &- 165. — Earthquake crack, Arizona. 
Earthquakes occur chiefly in regions which are still under- 
going movements of elevation and folding, and hence are 
intimately associated with young mountains and volcanoes. 
They are especially frequent along the borders of the 
Pacific Ocean, where the slopes of the continental plateau 
are steepest. In Japan noticeable shocks occur almost 
daily, and delicate instruments show that the earth-crust 
is in a continual tremor. 

Causes of Folding and Faulting. — The phenomena of 
folding and faulting evidently depend upon deep-seated 
conditions and forces in the interior of the earth. The 
thick beds of sedimentary rock which compose the mass 




192 



THE LAND 



of great mountain systems or rise high upon their flanks 
were certainly laid down in a nearly horizontal position 
upon the bottom of the sea. They have been not only 
elevated to their present position, but in the process 
of elevation have been extensively fractured, contorted, 
folded, and crushed in a manner which indicates that they 
have been subjected to enormous and prolonged lateral 
or horizontal pressure. 

The folded strata which underlie a certain area in Penn- 
sylvania sixteen miles wide would, if smoothed out to a 
horizontal sheet, extend ninety- six miles. The folding 
which has occurred in the Appalachian region has re- 
duced its original breadth about eighty-eight miles. 

Folded structure like that of Fig. 154 can be most 
easily imitated by laying a pile of sheets of rather stiff 
....,....-.-.•.•.,•. --...... ••.•..-..-. ■,--■- ■:•:■ paper upon a table and com- 
pressing them lengthwise. Fault- 
ing and other important details 
may be imitated by compressing 
layers of clay, plaster, or wax 
by means of a screw. 

Most of the typical displace- 
ments and deformations of strata 
may be easily and naturally 
accounted for by supposing that the earth-crust has in some 
way become too large for the centrosphere. Readjust- 
ment wider compression seems to be the key to most of 
the problems of mountain structure. Either the crust has 
grown larger than it was originally or the centrosphere 
has grown smaller. 

The theory that the earth has been for ages a cooling 
and therefore a contracting globe, and that while the 
crust on account of exposure to the heat of the sun has 




Fig. 166. —Clay compressed 
lengthwise. 



MOUNTAINS 193 

ceased to cool and contract the centrosphere continues 
to do so, is not free from objections and difficulties ; but 
in the present state of our knowledge it seems to be the 
most satisfactory explanation yet proposed. 

Regions like the Great Basin, which are traversed by 
faults, form exceptions to the rule that the earth-crust is 
under compression. As shown in Fig. 151, each fault 
block is wedge-shaped and those with a broad base have 
gone up while those with a narrow base have gone down. 
This is equivalent to the insertion of a series of wedges 
into the crust so as to enlarge it. The tilted blocks of 
the Great Basin act as if they were floating upon a liquid 
layer below, like blocks of ice upon water, which may be 
the actual case. It seems evident that in this region the 
earth-crust has been subjected to stretching instead of 
compression ; otherwise the blocks would not have found 
room to rise or sink between their neighbors. This may 
be accounted for by supposing that this portion of the 
crust forms the crown of a broad arch, and in crust fold- 
ing the crowns of the arches or anticlines are always put 
upon the stretch. 



CHAPTER XV 
VOLCANOES 




Fig. 167. — Stromboli. 

Stromboli. — The island of Stromboli rises from the 
Mediterranean Sea north of Sicily. It is a conical pile of 
material resembling cinders or the slag of an iron furnace. 

It is 4 or 5 miles in di- 
ameter and 3000 feet 
high. At a point on 
the steep slope about 
1000 feet below the 
summit there is a cir- 
cular hole from which 
a cloud of steam con- 
tinually escapes as if 

Fig 168. - crater of stromboli. from a chimney. It 

194 




VOLCANOES 195 

one climbs to a point which commands a view of the hole 
from above, the steam is seen to issue from cracks in a 
black crust which forms the bottom of a bowl-shaped hollow 
or crater. From some of the cracks steam is blown out in 
puffs with a loud snorting noise like that made by a loco- 
motive engine. In other cracks a thick semiliquid sub- 
stance heaves up and down, until finally a great bubble 
bursts with a rush of steam which carries fragments of the 
liquid several hundred feet into the air. At night the 
liquid is seen to be white-hot and the crust glows with a 
dull red color. Whenever the crust is broken by the burst- 
ing of a bubble, an incandescent surface is exposed from 
which the light flashes up on the steam cloud above, as 
it does when a locomotive fireman opens his furnace door. 

Stromboli is a volcanic cone and in its crater may be seen in a mild 

and simple form the essential features of a volcanic eruption. It will 

be observed that it is not a "burning mountain " ; in fact, there is next 

to nothing combustible in it. The appearance of flame above the crater 

is due to the illumination of the steam cloud by the white-hot lava, 

or melted rock, below. The lava in the crater acts very much like a 

kettle of mush, porridge, or molasses set over a fire. Steam is formed 

at the bottom, but can not escape readily on account of the thick, viscid 

character of the substance. The liquid boils and bubbles, gradually 

rising until it overflows the edge of the kettle or until a sudden and 

violent outburst of steam 

throws it in every direction. 

The island of Stromboli is 

entirely made up of material 

which has thus been expelled 

from the crater and piled up 

,.,„,. , , Fig. 169. —Section of Stromboli; 

around it. i he base 01 the 

pile rests upon the sea bottom, where the crater must have at first 

opened, more than 3000 feet below the surface. The eruptions of 

Stromboli are sometimes more violent than those just described, but 

they are always due to the formation of steam in a mass of melted rock 

and the explosion of the bubbles. 




196 



THE LAND 




Fig 170. 



Monte Somma; Vesuvius in the back- 
ground. 



Vesuvius is a conical mountain 4000 feet high, rising 
from a plain on the shore of the Bay of Naples. The 

upper part of the cone 
is half surrounded by 
a semicircular ridge of 
nearly equal height 
called Monte Somma. 
At the beginning of 
the Christian era this 
ridge formed part of. 
the wall of a complete 
crater about three 
miles in diameter, the 
bottom of which was 
occupied by a forest. In the year 79 this volcano suddenly 
burst into violent activity. The explosions blew away the 
south half of the crater rim, and scattered the material over 
the country, burying the cities of Pompeii and Herculaneum 
with mud and ashes. Since that time the present cone 
has been built up in- 
side the rim of the old 
crater. Vesuvius has 
long periods of rest, 
or of mild activity in 
which it resembles 
Stromboli. 

Globular masses of 
steam escape in rapid puffs 
and form a spreading cloud 
above the mountain. At 
the same time hot stones 
are hurled into the air, and 
fall back with a rattling sound like that of coal thrown into a cellar. 
The escaping gases, like the steam from the nozzle of a boiling tea- 




Fig. 171. 



Vesuvius and Monte Somma, as seen 
from Pompeii. 



VOLCANOES 



197 



kettle, are at first transparent, but change as they rise into bluish white 
fleecy clouds, while a peculiar " wash-day " odor is very noticeable. 
Small streams of liquid lava may be seen flowing down the side of 
the cone like liquid iron from a furnace. The surface of the lava soon 
cools and hardens into a stiff scum which becomes wrinkled like the 
cream on milk which is being poured from the pan. 

At irregular intervals the eruptions become much more 
violent. The explosions occur so rapidly as to make a 
continuous roar, the whole of the neighboring region is 
shaken, vast volumes of steam mixed with dust rise three 
or four times as high as the mountain, the cone is split by 
fissures from which 
lava streams flow, and 
the summit seems to 
sweat fire. Volcanic 
bombs, or whirling 
masses of lava, are 
thrown thousands of 
feet into the air ; the 
steam condenses into 
rain which is made 

dirty by the dust in Fig - I?2 - ~~ Vesuvius in eruption, April, 1872. 

the air and, mingling with the sand and stones, produces 
torrents of mud which overwhelm fields and villages. The 
eruption may continue for several days, but it gradually 
subsides, leaving the cone and crater changed in form and 
dimensions. 

These examples illustrate the essential feature of every 
volcanic eruption. It is the escape or expulsion of solid, 
liquid, and gaseous material from the interior of the earth. 
The hole or fissure from which the material escapes is the 
pipe or cliimney, the hollow around its top the crater, and 
the heap of materials piled around it the volcanic cone, or 
mountain. Steam forms more than ninety-nine per cent 




198 THE LAND 

of all the gases given off. The lava or liquid portion is 
simply melted rock, while the dust, often called "ashes," 
sand (lapilli), and larger stones or " cinders " are portions 
of lava blown into spray or clots by the explosion of steam. 
Pumice is a glassy lava which has been puffed up by steam 
bubbles into a light, spongy substance. Masses of lava 
having a coarsely cellular structure like bread are called 
scoria. Most of the volcanoes of the world resemble Vesu- 
vius in general character, but some are more violent, and 
others less so. 

Krakatoa. — The most violent and destructive volcanic eruption of 
modern times occurred in 1883 from the island of Krakatoa in the 
Strait of Sunda. During a series of explosions a mass of rock esti- 
mated at one cubic mile was blown into the air in the course of a few 
hours. A column of steam and dust rose to a height of seventeen miles 
and spreading out covered the sky with a black cloud which carried the 
darkness of midnight for scores of miles around. A rain of dust, sand, 
and fragments of pumice covered the sea and land. The noise of the 
explosions resembled that of heavy cannonading, and was heard at 
many places more than 2000 miles distant. The finest dust blown into 
the upper air was distributed by air currents all around the earth and did 
not completely subside for two or three years. Air waves were started 
which traveled three and a half times around the earth. The sea waves 
rose on the neighboring shores to a height of fifty feet, destroyed the 
lives of 35,000 persons besides a vast amount of property, and were felt 
on the shores of America 12,000 miles distant. About one half the 
island of Krakatoa was blown away, and in the place where a peak half 
a mile high had stood, the water is now 1000 feet deep. 

Hawaiian Volcanoes. — The island of Hawaii is the 
largest volcanic pile in the world. It is a mass of lava 
from 70 to 90 miles in diameter, rising from water 15,000 
feet deep to a height of 14,000 feet above sea level. 
There are four principal craters, of which two are now 
active. The summit of Mauna Loa, one of the highest 
points of the island, is a flat plain in the midst of which is 
a pit 3 miles long and nearly 2 miles wide and 1000 feet 



VOLCANOES 



199 



deep. This often contains a lake of lava thirty or forty 
acres in extent. From the surface of the lake columns of 
lava shoot up like a fountain to the height of several hun- 
dred feet. The lava seldom overflows the rim of the crater, 




Fig 173. —Map of Hawaii. 



but bursts through the side of the mountain at lower levels, 
spouting high into the air and forming a river of molten rock. 
After this the level of the lake in the crater subsides. 
On the eastern slope of Mauna Loa and nearly 10,000 



200 



THE LAND 



feet below its summit is another crater called Kilauea. 
This pit is from two and a half to three and a half miles 
in diameter. Near its center is a pool or boiling spring of 
lava. Part of the time this pool is covered with a black 
crust showing a rim of fiery liquid around its edge. Jets 
and fountains shoot up here and there, play for a few 
minutes, and subside. At intervals the whole crust be- 
comes broken by a 
network of cracks, 
each separate piece 
turns edge down- 
ward, and sinks, and 
the pool is left an un- 
broken expanse of 
glowing lava. Then 
the surface of this 
lake of rock freezes 
again. The pool is 
undoubtedly the top 
of a column of lava 
which extendsdown- 

Fig. 174.— Kilauea; lava lake. i ^i i r 

ward thousands 01 
feet. By repeated overflows of such lava pools the vast 
crater gradually fills nearly to the brim ; then, as the lava 
is drawn off through some subterranean outlet, its floor 
subsides until the pit is 1000 feet deep. 

- Volcanic vents like Mauna Loa and Kilauea may be thought of as 
springs or wells of liquid rock, the level of which varies with the supply 
and the facilities for escape. They are called calderas, or caldrons, 
and are distinguished by their great size, the extreme fluidity of their 
contents, the absence of violent explosions, and their habit of draining 
off at lower levels instead of overflowing. 

The lava streams which flow down the slopes of Mauna Loa are some- 
times half a mile to three miles wide and attain a length of forty-five 




VOLCANOES 



20I 



miles. When, as occasionally happens, a stream flows into the sea, the 
water is made to boil, the lava is shivered into fragments by the sudden 
cooling, and the air is 
filled with a fine glassy 
dust. The flow is at first 
very rapid and broken 
by cascades like those of 
a river, but as the gentler 
slopes are reached at 
lower levels, the stream 
spreads out, cools, stiff- 
ens, and is checked by 
its own want of fluidity. 
In flowing through for- 
ests it surrounds the 
trees, and may kill with- 
out destroying them. 




Fig 175 — Lava flowing into sea 

(Lava flow of iSSr, Hawaii. ) 



Islands of living forest are sometimes left in mid-stream. In highly 
liquid lavas like those of Hawaii a solid crust often forms over the sur- 
face while the liquid'm the interior drains away, leaving a long, winding 




Fig. 176. —Ropy lava, Hawaii. 

DR. FHYS. GEOG. — I 2 



202 



THE LAND 



tunnel or cavern. In other cases the crust breaks up into huge blocks 
which are carried along like cakes of ice in a river, and when the stream 



? 







3S> 



Mt. Shasta, California. 



Mt. Hood, Oregon. 







Mauna Loa, Hawaii. . Fujiyama, Japan. 

Fig 177— Profiles of volcanoes, showing slopes. 

finally solidifies are left in confused heaps which are difficult and dan- 
gerous to travel over, if not impassable. The surface resembles that 
of an ice gorge in a river or of the pack ice piled up by the currents in 
the polar oceans. If the lava stream is thin, the surface does not break 
up into cakes, but becomes wrinkled, ropy, and diversified with rounded 
projections, resembling a tangle of cables (Fig. 176). 

The slope of a volcanic cone depends upon the nature 
, ,, „ of the material. If the cone is made 

V</S\ CINDERS k SCORI.fi 

LAVA / \ 

ROCK BENEATH CONE 




Fig. 178. —Ideal section of a volcano. 

up chiefly of coarse cinders and scoriae, it is very steep. 
The slope of an ash cone is more gentle, while liquid lava 



VOLCANOES 



203 







spreads out easily into a still natter mound, the extreme 
being seen in the very liquid lavas of Hawaii, where the 
slopes are seldom more than seven degrees. Most volcanic 
mountains are of complex structure, being built up of suc- 
cessive layers of ash, cinders, and lava in roughly stratified 
arrangement. The strata slope away from the center and 
are bound together by radiating dikes, or nearly vertical 
sheets of lava which fill cracks made at times of eruption. 
Lava Flows. — The Columbia plateau, an area of 200,000 
square miles, mostly in Idaho, Washington, and Oregon, 
is formed by a series of 
lava sheets which in some 
places have a thickness 
of more than 4000 feet. 
The smooth surface of 
the lava meets the slopes 
of older mountains as the 
surface of the sea joins 
a rugged coast. It ex- 
tends up the valleys and 
indentations and is itself 
indented by projecting 

headlands, while Some Fig: 179 -Map of Columbia lava flow. 

mountains are completely surrounded and form islands in 
a frozen sea of lava. In some places it has been exten- 
sively eroded by streams, and in others broken by faulting 
into blocks, as in the block mountains of Oregon. 

Through the plateau the Snake River has cut a canyon which rivals 
in dimensions the Grand Canyon of the Colorado. From the exposed 
sections it appears that the hills, valleys, and mountains of the original 
surface were buried and obliterated by the lava flow, much as those of 
eastern North America were buried by the ice sheet. Buried soils, 
forests, and lake sediments, interbedded between the lava sheets, show 
that the outflow did not take place all at once, but at successive periods 








S, I UTAH 

i 

SCALE OF MILES 



260 ' ibo 



204 



THE LAND 




P^M**' 



Fig. 180. - The Devil's Slide, a dike. 
(Weber Canyon, Utah.) 



separated by. considera- 
ble intervals of time. The 
sedimentary rocks be- 
neath the lava are trav- 
ersed by numerous dikes 
which connect with the 
surface sheets, and indi- 
cate that the whole mass 
of the lava cap probably 
flowed out quietly from 
fissures in the earth- 
crust, and spread over 
the face of the country. 

A much older lava 
sheet of equal extent 
forms the plateau of 
Dekkan in India. 

Dikes, Sills, Lac- 
colites, Plugs, and 
Necks. — Cinder 
cones, beds of sedi- 
mentary rock, and 



other formations are often found to be traversed by nearly 
vertical sheets of hardened lava which clearly form no 
part of the original structure. The lava has been injected 
from below while in a liquid state, and has cooled without 
exposure to the air into 
a compact mass which 
shows no sign of hav- 
ing been permeated by 
steam. Such masses 
are called dikes. 



When the dike material 
is harder than the rock into 
which it was intruded, it is, 
by the process of erosion, 




Chasm formed by erosion of a dike. 
(Cape Ann, Mass.) 



VOLCANOES 



205 




left standing above the surrounding surface like a wall (Fig. 180). 
Where the dike penetrates harder rocks along the seashore, it is eroded 
by the waves more rapidly than the surrounding rock, and thus gives 
rise to " chasms " (Fig. 181), "purgatories," and spouting caves. 

Lava has been forced not only into vertical cracks, but 
also between the layers of stratified rock, where it has 
hardened and forms 
more or less horizontal 
bills. In some instances 
the quantity of liquid 
rock forced into one 
locality is sufficient to 
raise the overlying strata 
into a dome which ap- 
pears upon the surface as a mountain. Such accumula- 
tions of igneous material in the midst of sedimentary rocks 

are called laccolitcs 
{stone cisterns). Sub- 
sequently they may 
be exposed by the 
erosion and removal 
of the overlying 
strata, as is the case 
Fig. 183. —cross section of Mt. miiers. in the Henry Moun- 

tains of Utah. The laccolite of- Mt. Hillers in this group 
is three miles in diameter and 4000 feet deep (Fig. 183). 



Fig. 182. — Ideal cross section of a laccolite. 



\8tratifed Rock a ,n)ttD' 





5 4 3 

Fig. 184. — Cross section of Black Hills. 

The Black. Hills of South Dakota are the eroded remains of a dome 
which if complete would be from 80 to 160 miles in diameter, and 7000 



206 



THE LAND 



Fig. 185. 



Cross section of Elk Mountains, Colorado. 

(G, granite.) 



feet high. The cen- 
tral core is of gran- 
ite, from which the 
strata of sandstone, 
shale, and lime- 



Fig. 186. — Cross section of Park Range, Colorado. 
(B, lava; M'G, granite.) 




Fig. 187. —Hogbacks (after Powell). 
(Northern slope of Uinta Mountains.) 

Rocky Mountains in Colorado seem to 
of strata by an intruded 
core of granite (Figs. 185, 
186). The flanking ridges 
of hard strata are called 
"hogbacks 11 (Fig. 187): 
In a few instances the 
lava has been upthrust in 
the form of a vertical col- 
umn which is afterwards 
exposed by erosion and 
is known as a volcanic 
plug. Mato- tepee in 
Wyoming is a plug 600 
feet high (Fig. 188). In 
the last stages of the de- 
struction of a volcanic 
cone by erosion, the core 
of hardened lava which 
filled the original pipe or 
chimney at its center is 



stone dip away in every direc- 
tion. The edges of the hard 
strata form ridges which en- 
circle the core, and between 
them the softer strata have 
been eroded into valleys 
which are continuous around 
the Hills. The Elk, Park, 
and other ranges of the 
be the result of similar upth rusts 





Fig. 188. — Mato-tepee, Wyoming, a plug. 



VOLCANOES 



207 




left standing alone after the 
removal of the other material, 
and forms a lofty butte which 
resembles a plug but is called 
a neck (Fig. 189). 

Dust Deposits. — The quan- 
tity of so-called " ashes " or 
dust emitted by volcanoes is 
sometimes so great as to form 
beds comparable in extent 
and importance to the lava 
sheets. Whymper saw a 
column of inky black dust 
rise from the crater of Coto- 
paxi, Ecuador, straight up to 
a height of 20,000 feet in less 
than one minute, and then spread out with the wind. The column con- 
tinued to rise for more than an hour, and the dust was distributed over 
hundreds of square miles. The quantity which fell was estimated to be 
not less than two million tons. The fall of sand from an eruption of 
Conseguina, Nicaragua, in 1835 spread over an area 1500 miles in diame- 
ter, and continued for forty-three hours. Near the volcano the country 
was buried to a depth of several feet, and the deposit was several inches 
deep 100 miles away. Extensive deposits of volcanic dust, the result of 
eruptions which occurred in long past ages, extend over large areas in 
Nevada, Utah, Montana, South Dakota, Nebraska, Oregon, and Washing- 
ton, attaining in many places a thickness of 30 to 50 feet. Along the 
Yukon River, a bed of white volcanic dust covers an area of 50,000 
square miles, varying in thickness from a few inches to 100 feet. 



Fig. 189. — A neck. 
(Near Mt. Taylor, N.M.) 



Distribution of Volcanoes. — Eruptions of material from 
the interior of the earth have occurred at some period in 
almost every part of the world, but as Fig. 190 shows, 
recent volcanic activity is almost confined to regions of 
young and growing mountains, and is especially marked 
along the high margins of the continents which border 
the Pacific and Indian oceans, and among the peninsulas 
and islands in tropical and equatorial regions. 



208 



THE LAND 




■ Present or Recent Action (Cross fines shout known Activity} m *T 

- Generally Ancie nt (C.-^.s.'i sliou' u'lvc Activity continues to the Present) 



Fig. igo. — Map showing distribution of volcanoes. 

Causes of Volcanoes. — The most impressive and terrify- 
ing displays connected with volcanic eruptions are plainly 
due to the explosion of steam within the crater ; but this 
does not account for the origin or existence of the liquid 
rock, nor for eruptions which are not explosive, like those 
of Hawaii and the lava flows of the Columbia plateau and 
the Dekkan. It is necessary also to account for the 
steam itself. 

The fact that volcanic eruptions generally occur in 
regions where elevation and folding have recently taken 
place or are still going on, indicates that both upheaval and 
volcanism may be due to the same general cause. 

If the rock at a depth of twenty or thirty miles below 
the surface of the earth-crust (see p. 28) is hot enough 
to melt, but is kept solid only by pressure, it seems proba- 
ble that a comparatively slight relief from pressure would 
be followed by immediate liquefaction. The upfolding of 



VOLCANOES 209 

the strata above, or the occurrence of a fissure extending 
downward from the surface, would give the rock below 
more room; the pressure at that point would diminish, 
and, driven by the greater pressure all around, the now 
melted rock would rise. The water which the rock origi- 
nally contained, or which it might meet on its way upward, 
would expand into steam, and still further assist the rise 
of the lava column, as the steam formed in a kettle of boil- 
ing mush or molasses causes it to rise and overflow. If the 
lava is thick and viscid, the steam escapes with explosive 
violence ; if it is thin and liquid, the steam passes off more 
quietly. 



CHAPTER XVI 
LAND SCULPTURE 

The forces at work within the earth have roughly shaped 
the large features of its face, — the ocean basins with their 
level floors, rounded deeps, broad rises, and curving lines 
of elevation, the continents with their plains, plateaus, 
mountain systems, closed basins, volcanic cones, and lava 
beds. One fourth of the face has been exposed to the 
action of outside agents which have covered these huge 
and unshapen forms all over with elaborate carving and 
given them an endless variety of detail. Air, frost, 
gravity, and running water are great artists, who sculpture 
in stone their characteristic patterns of ornamental design. 
Each pattern is the result of a different combination of 
agents at work upon different materials. The whole as- 
semblage of designs forms the foundation of scenery or 
the diversified landscapes of the earth. 

The character of sculptured forms depends upon (i) the agents, such 
as rainfall, temperature, and winds, which vary with the climate : and 
gravity, which is everywhere practically uniform in force, but not every- 
where equally efficient. The work which gravity can do. either directly 
or through running water, varies with the steepness of the slope. On 
level plains it accomplishes hardly anything ; on sharply inclined moun- 
tain sides and perpendicular cliffs its efficiency is very great. 

The character of sculptured forms also depends upon (2) the mate- 
rials, which may be soft or hard, brittle or tough, porous or compact, 
homogeneous or varied in texture, coarse-grained or fine, soluble or 
insoluble ; and upon (3) the structure, or arrangement of materials, 
whether stratified or unstratified, whether the strata are horizontal, in- 
clined, or folded, and whether the hard or soft strata are uppermost. 
If there were anywhere on the face of the earth a region upon which 

210 



LAND SCULPTURE 



211 



no rain ever falls, over which no wind ever blows, subject to no changes 
of temperature, and so level that gravity is powerless, it would undergo 
no change from age to age, and would be truly dead. No known region 
even approaches such a condition. 

Sculptured Forms in Horizontal and gently Inclined 
Strata. — The plain south of the Great Lakes presents one 
of the simplest of sculptured designs. Over a large part 




Fig. 191 — Shallow valley in a drift plain 

of it the slopes are scarcely perceptible, the material is im- 
perfectly stratified, varies but little in hardness, and is not 
affected by changes of temperature. The rain soaks into it 
readily and the run-off is sluggish. In the course of time 
the streams cut broad trenches a little way into it and 
develop meanders and flood plains out of all proportion to 
the volume of water in the channel. Except for these 
shallow stream valleys and the low bluffs which border 
them, the landscape is featureless (see Figs. 97, 191). 



212 



THE LAND 




Fig. 192.— The Bad Lands. 

(Washington County, S.D.) 



Similar conditions prevail upon coastal plains recently ele- 
vated above the sea, and in filled valleys and lake basins. 
A smaller rainfall and greater elevation produce 1 out of 

quite similar material 
the elaborate and 
complex forms of the 
" Bad Lands " of 
South Dakota and 
Nebraska. There a 
lake basin has been 
filled with nearly 
homogeneous, com- 
pact clay, occasion- 
ally varied by thin 
beds of sandstone 

and slightly harder clay. This originally smooth plain has 

been uplifted, and during the progress of the uplift the 

neighboring rivers 

and their growing 

branches have cut 

their valleys deeply 

into the surface, until 

it has been sculptured 

into narrow ridges, 

steep-sided buttes, 

sharp cones, isolated 

towers, pinnacles, and 

castellated forms in 

endless variety. Some 

small areas of the original surface remain as flat-topped 

mesas (see p. 214), which are bounded by high, rugged 

walls, notched by canyons, and buttressed by projecting 

spurs. 




T ig 103 — The Bad Lands. 
(Sioux County, Neb.) 



LAND SCULPTURE 



21 



Lands 
larger 



Many of the pinnacles owe their form to a sandstone cap which 
protects the underlying clay (Fig. 194). The mesas are covered with 
sparse grass, but the slopes are bare and brilliantly colored. The 
rainfall is so small that convex slopes are rare and short, the descent 
from the level summit being abrupt and steepest at the top (see p. 156). 
A group of such forms 
bears a strong resem- 
blance to a gigantic city 
in ruins. 

The Colorado pla- 
teau region described 
in Chapter VI pre- 
sents forms similar to 
those of the Bad 
on a much 
scale. The 
rocks are, for the 
most part, horizon- 
tally stratified, and 
present considerable 
variations of texture 
and hardness. The 
activity of the streams 
consequent upon the 
great elevation of the 
country has carved 
out characteristic 
flat-topped forms of 
mountainous size. Where the strata are thick-bedded, com- 
pact, and of uniform hardness, vertical cliffs a thousand 
feet high occur. Where the strata are thinner and variable 
in hardness, there is an alternation of vertical steps and 
more or less gentle slopes. Extensive outflows of lava 
have covered portions of the plateau with a protective cap. 




Fig. 194. — A capped tower. 
(Vulcan's Anvil, Monument Park, Colo.) 



214 



THE LAND 




Fig. 195. — Lava-capped mesa. 

(Raton Mesa, Colo.) 

Where the underlying rocks have been undermined by the 
lateral corrasion of the streams, an isolated block with ver- 
tical sides and a flat, sometimes overhanging, top of lava 
is left standing. These form typical mesas (Spanish for 
" tables "). 

In the northern part of the Colorado plateau region the strata are 
gently inclined to the northward, and form a series of tilted steps 
each of which slopes down to an escarpment or line of cliffs (Fig. 
196). The drainage is down the slope and along the foot of the cliffs 
to the trunk stream which cuts through the plateau (see p. 84). The 

escarpment is gradually under- 
mined, and made to- retreat 
down the slope. At the same 
time it is notched by canyons, 
which often meet at their heads and surround a portion of the cliff cut off 
from the rest. Such isolated outliers form typical buttes, which stand in 
front of the cliffs like a line of sentinels, and present every degree of sepa- 
ration from the main mass of which they were once a part (Fig. 197). 



Fig. 196. 



Fig. 197. — Pink Cliffs and outlying buttes, southern Utah. 




Fig. 198. — Land of Standing Rocks, near Cataract Canyon, Utah 
215 



2l6 



THE LAND 




Outlying fragments of a former terrane 1 often occur many miles from 
the nearest cliff or outcrop of similar strata, and furnish a means of 

determining the amount of de- 
nudation or removal of strata 
which has occurred. In Fig. 
199 the butte D and the mesa 
Flg ' *"■ M are composed of the same 

strata in the same order as the escarpment E, and it is evident that 
the strata once extended across and filled the intervening spaces. 
By such evidence it is shown that the average thickness of strata re- 
moved from an area of 15,000 square miles on the Colorado plateau 
can not be less than 10.000 feet. 




J FARSpffiffi M R H I A D \ 





Fig. 200. — A young plain. 
(Red River basin, N.D, and Minn.) 



"VtT? 



.) X I 








Fig. 201.— A dissected plateau. 
(Tioga and Potter counties, Pa.) 



Development of Sculptured Forms. — The sculpture of 
a plateau of horizontal or gently inclined strata presents 

1 Any mass or series of rocks of large extent is called a terrane. 



LAND SCULPTURE 217 

several recognizable stages of progress. (1) Its origi- 
nally smooth surface is the result of its structure and con- 
forms to the surface of the upper stratum. The region 
is young and the landscape is featureless (Fig. 200). 
(2) The streams cut deep trenches into it and their tribu- 
taries extend their head waters in every direction. The 
plateau is cut by a network of water channels into a group 




Fig. 202. —A peneplain, northern Virginia. 

of ridges and blocks of very irregular size and shape. The 
tops of most of them are flat and their sides steep, while 
the valleys are narrow and V-shaped. (3) As the streams 
widen their valleys the ridges grow narrower until they 
are sharp-crested, and the blocks grow smaller until they be- 
come conical, pyramidal, or dome-shaped. The ascent out 
of one valley to a divide is immediately followed by a de- 
scent into another. The originally smooth surface has been 
changed into a surface of the utmost possible roughness 
and the plateau is said to be maturely dissected (Fig. 201). 

DR. PHYS. GEOG. — 14 



2i8 THE LAND 

(4) As erosion continues, the ridges and hills grow smaller, 
lower, and more rounded, until all abrupt and prominent 
irregularities disappear, and the once elevated plain, re- 
duced to a gently undulating, low-lying peneplain 1 (Fig. 
202), returns in its old age to the nearly featureless con- 
dition of its youth. 

Sculptured Forms in Folded Strata. — Any system of 
rock folds, however complex, consists essentially of a series 

of arches and troughs 

^^^^^^^^s^^^^^^^. (anticlines and syn-. 

-^^^^^^^^^ ^ ^^^^^^^ clines). In the process 

Fig. 203. — Eroded anticline. . . , , . ., . , . 

of folding, the anticlines 
have been subjected to a stretching force and are more or 
less broken at the crown of the arch. They are also ele- 
vated into regions where frost and changes of temperature 
are more efficient ; 
hence anticlines are 
especially subject 
to rapid weather- 
ing and disintegra- 

„, . Fig. 204. — Eroded syncline. 

tion. The mantle 

rock is easily removed down their slopes, and, accumula- j 
ting in the structural valleys, partly protects the synclines 
from erosion (see Fig. 1 54). The harder and softer strata 
of the anticline are affected unequally, and the form re- 
sulting from the process of sculpture is usually a plateau 
traversed by parallel ridges and valleys (Fig. 203). But 
with prolonged erosion anticlines are reduced to valleys 
and synclines remain as ridges (Fig. 204). These occur 
frequently in the Appalachian highland (see Fig. 155). 

The forms which result from the sculpture of highly 
folded, contorted, and overthrust rocks are so complex and 

1 Pene, almost, and plain. 







LAND SCULPTURE 



219 



confused as to defy classification. Some examples are 
given in Figs. 162, 205, 206. They present irregular groups 
and series of jagged crests and sharp peaks separated by 




Fig. 205. — Alpine sculpture. 

narrow and profound valleys, the surface of which does 
not correspond to the form of the folds. 

In every mountain system the valleys may be distinguished as in 
part structural, or due to the original folding, and in part sculptured, 
or due to erosion. The structural valleys are longitudinal, extending 
lengthwise between the ranges, and have usually been greatly modified 
by erosion. Every stream which flows down the slope of a mountain 
range cuts a transverse valley, which frequently extends from crest to 
foot. The ridges between the transverse valleys form spurs extending 
at right angles to the main crest (see Fig. 148). When two streams 
on opposite sides of a range have their head waters near together, in 
the course of time the divide is 
eaten away until there is cut in 
the crest line a notch which may 
extend downward to one fourth 
or one third the height of the 
range. These notches are called 
passes, and afford the easiest 
routes of travel across the range. 
Thus the crest may become di- 
vided into a series of passes and 
peaks. 

Mountain ranges, like plateaus, 
are subject to denudation and final destruction by the agents of erosion.' 
The Height of Land between the Great Lakes and Hudson Bay was 
once a mountain range which may have been as lofty as the Alps, but 




Fig. 206. — Alpine sculpture. 



220 



THE LAND 



has been broken and ground down by weathering and glacial abrasion 
until it is a broad, plateaulike ridge conspicuous only by its influence 
upon the direction of drainage. The plateau of southern New England 
consists of the stumps of old mountain ranges which have been reduced 
to a peneplain and reelevated. Only the folded and disturbed position 
of the rocks and an occasional projecting knob of hard material, like 
Monadnock and Wachusett, remain to indicate that mountain ranges 
once existed there. 

Glacial Sculpture. — The effect of glacial action upon 
the form of the land has been considered in Chapter X. 
The general tendency of glacial abrasion is to rub down 



:^J*|JjIOjI 


2§S 


A,. 


"^S 


A . 



Fig. 207. — Glaciated profile. Fig. 208. — Unglaciated profile. 

(Carey Island.) (Dalrymple Island.) 

(Two islands of like geological structure, near Greenland.) 

projections and angles, to gouge out soft rocks, and to 
produce a surface of rounded knobs and hollows. The 
contrast between a glaciated and an unglaciated surface is 
often very sharp and conspicuous. 

Mountains in high latitudes are generally less jagged and have more 
smoothly flowing outlines than those in temperate and tropical regions. 
The passage of a glacier through a stream valley changes its form from 
V-shaped to U-shaped. Whether glaciers exert a scooping action and 
excavate valleys to a greater depth in one place than another, thus 
forming rock basins, like those of the Finger Lakes (pp. 140-142), is a 
question which has been earnestly discussed. The evidence seems now 
conclusive that they have done so. Many valleys in the Alps, Himalayas, 
Rockies, Sierra Nevada, and other glaciated mountains contain lake basins 
which can hardly be accounted for by any other process than glacial 
abrasion. Striking proof of the extent of glacial abrasion is furnished 



LAND SCULPTURE 



221 



by " hanging valleys," or tributary valleys which enter a main valley at 
heights far above its floor, thus seeming to end in the air, as in Fig. 106. 
These regions also contain peculiar cirques or " amphitheaters " with 
precipitous walls curving around in a semicircle. These occur at the 
heads of gorges, at the meeting point of many little streams which flowed 
down the steep mountain side and cut little gorges separated by spurs. 




Fig. 209. — Avalanche Basin, Montana, a cirque. 

These were afterward occupied by convergent neve fields and ice streams 
which crowded together into one main glacier. The ice cut away the 
spurs and gouged out the bottom, thus producing the semicircular basin. 

Wind Sculpture. — Sand driven by the wind is a very 
efficient sculpturing tool and in desert regions accom- 
plishes important results. Hard rocks acquire a highly 
polished surface, while rocks of uneven texture are etched 
into patterns which leave the harder portions standing out 
in relief. The currents are irregular in force and direc- 



222 



THE LAND 



tion and grind out curious hollows and niches. Exposed 
rocks are everywhere grooved, fretted, and honeycombed 
(Figs. 30, 210). The bulk of the sand is lifted only a 

few feet, hence cliffs 
are attacked most 
rapidly at the bot- 
tom, and the upper 
parts are left over- 
hanging. One strik- 
ing peculiarity of 
wind erosion is the 
absence of talus 
slopes, the 



fallen 
being 
swept 




fragments 



Fig 210. 



Rock eroded by wind and sand 

(Near Laramie, Wyo.) 



continually 
away, leaving the 
cliffs bare to the base. The final result of wind erosion 
is to level the larger inequalities and to leave a plainlike 
surface strewn with polished boulders, which on account 
of superior hardness have survived the destruction of the 
parent ledges. 

General Summary. — The general process of develop- 
ment of sculptured forms is modified in a thousand details 
by minor variations of climate, material, and structure. 
The rapidity of development increases with the rainfall 
and with the softness and incoherence of the material. 
In arid regions all forms are more angular and sharply 
defined, in moist regions more rounded and indefinite. 
Tough and compact rocks sustain themselves at sharper 
angles, in steeper slopes, and* in higher cliffs, than loose and 
friable rocks. The faces of cliffs are modified by the 
presence or absence of joints or cracks which traverse 
most rock beds in various directions. Thick-bedded lime- 



LAND SCULPTURE 



223 



stones and sandstones 

often present two sys- 
tems of nearly vertical 

joints, which divide the 

beds into large rec- 
tangular blocks, giving 

them the appearance of 

courses of cyclopean 

masonry. These joints 

determine the planes 

along which the rocks 

split easily, so that the 

face of a cliff may be 

vertical or overhanging, 

according to the angle 

of jointing. The loftiest 

and most precipitous 

slopes occur in rocks 

which are practically 

free from joints, as in the 3300-foot granite face of El Capi- 

tan in the Yosemite Valley. In the course of weathering 

small local differ- 
ences in the quantity 
of cement in a rock 
are etched out into 
curious patterns of 
fretwork, as shown 
in Fig. 211. 

There is one law 
of universal applica- 
tion to sculptured 
surfaces, — the Jiard- 
Fig. 212. — ei capitan. est survives. The 




Fig. 211. 



Jointed rocks and differential 
weathering. 




224 THE LA ND 

level surface of a denuded plateau or mesa is generally the 
top of a hard bed, from which softer beds have been re- 
moved and upon which erosion has come to a comparative 
standstill. Streams are apt to avoid obstacles and to cut 
their valleys along lines of least resistance. Broadly speak- 
ing, elevated regions are, as a rule, composed of relatively 
resistant rocks, and depressions of relatively unresistant 
rocks. When the work of erosion is nearly completed, 
every ridge, crest, peak, spur, mesa, butte, knob, and dike 
stands above the average level because the material is 
harder ; and nearly every valley, basin, gorge, clove, notch, 
pass, and gap sinks below the average level because the 
material is softer. 

Influence of Mountains on Life. — The conditions upon 
mountains are so different from those of the surrounding 
lowlands that they are in many respects like islands in 
the sea. The upheaval of the earth-crust and subsequent- 
erosion have brought to the surface many rocks and min- 
erals not usually found elsewhere. Weathering goes on 
rapidly, but on account of the steepness of the slopes 
mantle rock does not accumulate. Streams are small, 
rapid, and. not navigable. The ranges and peaks rise 
through the warm, dense lower air, and penetrate the 
upper air, which is cold, thin, and dry. Hence the tem- 
perature may vary as much from the base to the top of a 
mountain as from the equator to the pole. On the side 
toward the sea the rainfall is apt to be heavy, and on the 
land side to be light. This variety of conditions makes it 
possible for a great variety of plants and animals to exist 
in a small area. In some places it is possible, by ascending 
a mountain, to pass in one day from the luxuriant vegeta- 
tion and abundant animal life of a tropical forest to a region 
of perpetual ice and snow. Agriculture, commerce, and 



INFLUENCE OF MOUNTAINS ON LIFE 225 

human settlement often extend up the lower slopes of 
mountains. Above these is a region of forest and pasture, 
where population is sparse, and herding, lumbering, and 
mining are the chief occupations. 

Mountains are formidable barriers to the migration of 
plants, animals, and men ; hence the vegetation, animal life, 
and people on opposite sides of a mountain range are often 
very different. The inhabitants of mountainous countries 
are, as a rule, inferior in some respects to the people of the 
plains. In case of invasion and conquest, the victors occupy 
the rich and productive lowlands, while the conquered take 
refuge in the mountains. There, on account of the diffi- 
culty of access and communication, they remain isolated 
and cut off from the general progress of the world. Sparse- 
ness of population and the hard conditions of life are un- 
favorable to the development of the arts of civilization. 
Primitive customs and habits are preserved, and the moun- 
taineers remain relatively rude and uncultivated. At the 
same time, the conditions are such as to render them brave, 
hardy, industrious, and hospitable. They love and maintain 
political freedom, and escape the corrupting influences of 
crowded centers of population. 

Lofty mountain ranges have commonly been regarded 
with abhorrence, as being deformities and blemishes upon 
the face of the earth, and the abode of evil spirits. From 
the time of the Romans until about a century ago, the Alps 
were the most efficient barrier to travel in Europe. No one 
visited them unless compelled by the necessities of war or 
commerce. A journey across them was regarded as a 
hardship to be avoided or endured. Interest in the lessons 
which mountains have to teach concerning the structure 
and history of the earth, and appreciation of the beauty 
and grandeur of mountain scenery, had their principal 



226 THE LAND 

birthplace in the Alps, and are chiefly the product of the 
nineteenth century. To-day few regions in the world 
have so many visitors for pleasure, recreation, and study. 
Switzerland and the Alps have become the playground of 
Europe. In the United States, the Appalachian and 
Rocky Mountains vie with the seashore in attraction for 
tourists and visitors. Every summer, thousands of people 
go to the mountains to enjoy pure air, cool temperature, 
healthful exercise, and the inspiration of wild and beau- 
tiful scenery. 



CHAPTER XVII 

COAST FORMS 

"The sea is not satisfied with an irregular shore line, and in its 
attempt to reduce the land to a submarine platform it will straighten 
the shore line in order better to attack the land. 1 ' — Gulliver. 

As the surface of the land is being continually changed 
by the action of the atmosphere and running water, so 
the restless sea is at work upon its edge, gnawing away 
in one place and building up in another. Hence coast 
lines bordering upon large bodies of water present a series 
of characteristic forms. The ocean basin and the conti- 
nental block seem to have been permanent features from 
a very early period of the earth's history, but the belt on 
each side of the seashore has been called the variable 
zone on account of its frequent changes of level. There- 
fore coast lines, like relief, are primarily determined by 
the forces of upheaval and depression, and rising coasts 
and sinking coasts are distinguished by strongly marked 
characteristics. 

Rising Coasts. — Along a rising coast the sea retreats 
and the new shore line is formed upon smooth sea bottom ; 
it is comparatively regular, and may extend for thousands 
of miles in a series of long, gentle curves. 

Sharp, narrow projections and indentations are absent, and there 
are seldom small bordering islands close to the shore. Such coast lines 
are often bordered by mountain chains, and either the slope from the 
mountain crest to the deep sea floor is abrupt and continuous, or only 
a narrow coastal plain intervenes. The Pacific coast of America be- 
tween Puget Sound and latitude 40 south, and the whole coast of Af- 
rica are conspicuous examples of young and rising coasts. 

227 



228 



THE LAND 



Sinking Coasts. — Along a sinking coast the sea advances 
and the new shore line is established upon the sculptured 
surface of the" land ; hence it is irregular. The sea ex- 
tends up every valley, converting it into a bay or estuary. 
Between the bays the ridges project into the sea as capes, 
promontories, and peninsulas. Numerous elevations are 




Fig. 213. — Lynn Canal, Alaska, a fiord. 

surrounded by water and converted into islands, which 
stand in front of the mainland coast. Sinking coast lands 
may be in any stage of sculpture and dissection ; hence 
their shore lines present a varied irregularity. 

The subsidence of the old denuded plateau of northeastern North 
America has broken it up into the islands of the Arctic archipelago and 
let the sea into the great interior basin of Hudson Bay. Newfoundland 
is cut off by the drowned valley of the St. Lawrence, which the sea 
has invaded to a distance of five hundred miles. Upon the coasts of 



COAST FORMS 



229 



Norway, Alaska, and southern Chile, deeply dissected mountain ranges 
have sunk about half their height and let the sea into their very heart. 
They are now pierced by long, narrow fiords and canals, which are of 
great depths and bounded by lofty and precipitous walls (Fig. 213). 
These are usually shallower at the mouth and deeper inward. They 
resemble glaciated mountain gorges and are a joint product of stream 



CASCO BAY 

MAI5E 

SCALE OF MILES 




Fig. 214. —Part of the Maine coast. 

and glacial erosion . Glaciated plateaus which border upon the sea, like 
those of Greenland and Maine, also present a fiord coast of the most 
ragged outline. The firths, sea lochs, and islands of the coast of Scot- 
land are due partly to glaciation and drowning and partly to the action 
of waves upon rocks of varied material and complex structure. 

Wave Action. — Coast lines which remain stationary at 
the same level for a sufficient length of time are modified 



230 



THE LAND 




by the action of waves, currents, and tides. Waves are 

the chief agents of disintegration and sculpture, while 

tides and currents perform the work of transportation, 

corrasion, and deposition. The crest of a wave in shallow 
water moves forward with great velocity and 
strikes a blow against the shore. The water 

^j^r^^—— _ carried forward in the 

zzzBgj^Jj^^ crest returns along the 
F ig 215. bottom in a current 

called the undertow. By constant repetition of this action, 

the edge of the land is crumbled away and a cliff is formed 

which varies in height with the elevation of the coast. 
The fallen fragments are lifted, rolled 

over one another, and even hurled back 

against the cliff, until they are ground fine 

enough for the undertow to carry away. 

The erosive action of the waves, like that 

of a river, is greatly increased by the sedi- 
ment which they carry. The dragging of 

the sediment back and forth 

over the bottom corrades the 

sea floor down to a level some- 
what lower than the troughs 

of the waves. Thus waves 

act like a horizontal saw which 

makes a wide cut into the 

land, extending above and 

below the level of still water. 

The portion of the land above 

the cut breaks away and forms 

a sea cliff, along the foot of 

which stretches a level or 

gently sloping submerged 

platform or wave-cut terrace 

(see Fig. 215). The surface 

of the terrace is often diversified by small islands, reefs, and skerries, 

fragments of the cliff which were hard enough to resist erosion. In 




Fig. 216. — A stack. 



COAST FORMS 23 1 

this process the relative hardness of the rocks plays an important part. 
A streak, vein, or dike of softer rock is removed more rapidly, and a 
ravine or tunnel is cut into the cliff by the waves. The ravines some- 
times intersect, and the intervening harder blocks are separated from 
the main body of the cliff and left standing as sea-made buttes or stacks 
(Fig. 216). 

Shore Drift. — Winds and waves seldom strike the shore 
at right angles, and when they strike obliquely currents 
are set up parallel with the trend of the coast. Such cur- 
rents are intermittent, irregular, and often reversed in direc- 
tion, but there is usually some prevailing direction in which 
the waste of the land is carried. Thus the shore drift is 




Fig. 217. —A beach, Mackinac Island, Michigan. 

distributed all along the coast in a zone called the beach 
(Fig. 217). The material may be. sand, gravel, or boulders, 
and is always well rounded and partly assorted. 

The beach lies partly above and partly below the level of still water, 
and its profile is a double curve, convex at the top and concave toward 
the bottom. 

Where the sea bottom has a very gentle slope, the waves break at 
some distance from the shore and the sand they carry is deposited, 
forming a barrier beach (B, Fig. 218) along the line of greatest dis- 
turbance. The strip of water inclosed between the land and the barrier 
beach is called a lagoon (L, 
Fig. 218). It is often partly 
filled with stream sediment 

or wind drift and converted 

,., , , , Fig. 218. 

into a tidal marsh. In some 

cases the shore current builds up the beach above sea level and adds 

to it on the seaward side, so that it gradually widens into a wave-built 




232 



THE LAND 



terrace or foreland (see T, Fig. 219), the surface of which is traversed 
by parallel ridges, each the mark of some exceptionally violent storm. 

Where the current turns 
away from the coast, the 




r^^^-r^r: foreland is built out into 



Fig. 219. — Section of a wave-built terrace 



a blunt point or cusp, as in 
Fig. 220. 

The shore current does not 
follow all the bends of the 
shore line, but sweeps across 
the mouth of a bay, carrying the shore drift with it 




Fig. 220. 



Thus a bar or 

continuation of the beach is built out from one point and sometimes 

extends to the opposite point, 
completely closing the bay. 
Usually, at some place in the 
bar, a channel is kept open by 
the tide or the current of a 
stream which empties into the 
bay. If the bar extends only 
part way across the mouth of 
the bay, it is called a spit, and 
when the end of the spit is re- 
curved by some current in a 
different direction, it becomes 
a hook (Figs. 222, 223). A 
bar may extend from the shore 
to an island, thus tying it to the 
land (Fig. 224), and in some 
cases there are two or more 




Fig. 221.— Map of bars on Lake Superior. 



connecting bars. The beach, bar, spit, and hook are only slight va- 
riants of a single form. They are at once accumulations of shore drift 
and roads along which shore drift is traveling. Their manner of con- 
struction is similar to that of a railroad grade along which material is 
transported for its own extension. 

The Graded Plain. — The tendency of wave and current 
action is to cut back projecting headlands and to build 



COAST FORMS 



233 





J 



Fig. 222. — A hooked spit. 

bars across the bays, thus producing a smooth shore line. 
If a condition could ever be reached in which the head- 
lands were all cut back and the bays all filled, the coast 



Ca/w Cod 




if 

:Sp Pr ovincetoi 
1 Harbor 




BAY 



Fig. 223. —The end of Cape Cod, Massachusetts, a hooked spit. 

DR. PHYS. GEOG. — 15 



234 



THE LAND 




line would be mature, the 
various forces would be 
perfectly adjusted, and 
from that time onward 
the sea would expend its 
energy in sawing into the 
land and reducing it to a 
submarine plain. In the 
mean time the reduction 
of the land surface to 
base level would be going 
on, and the product of the 
land - destroying forces 
would be a graded plain, 
lying partly above and 
partly below sea' level, 
as shown in Fig. 225. 

The land portion of a graded plain is gradually completed by atmos- 
pheric forces acting upon all parts of its surface at once, while the sea 
portion of the plain is com- 

pleted strip by strip, once for ' • ™*^"->"" i ^ — ■■■> 

all, as far as it goes. Most ...,\ 

of the forms that are due to 
atmospheric forces are char- 
acterized by relief, elevation, verticality ; most of the forms due to 
marine forces are characterized by horizontality. 

Deltas differ from all other coast forms in being the 
work of land streams, which build them in spite of the 
opposition of waves, tides, and currents. The work of a 
river is to convey its load of sediment to the sea and to 
push forward a delta lobe for each distributary. The 
work of the sea is to destroy this irregularity in the coast 
line, to cut off the lobes, and to carry the material to a 
position of rest. 



Fig 224. -Island and bar. 




Fig. 226. — Blocked rivers. 



COAST FORMS 235 

In most cases the sea is stronger than the river, and well-developed 
deltas are exceptions rather than the rule. A stream which empties into 
a drowned valley builds a delta at the head of the bay and may in time 
fill it, thus assisting the sea in. its efforts toward a smooth coast. The 
Mississippi delta is a typical form in which the river forces are entirely 
superior to those of the sea (Fig. 40). Many varieties, more or less 
lobed, rounded, pointed, or 
straight, may be found be- 
tween this extreme and the 
opposite where the river is 
entirely overcome by the sea. 
In such cases the river is 
turned aside and compelled 
to flow a long distance parallel 
with the beach before finding an outlet, or its mouth is completely 
blocked by a straight bar, and the water escapes by percolation through 
the sand. 

Effect of Tides. — The general direction of tidal forces 
is at right angles to the coast line, and therefore at right 
angles to the alongshore currents which build beaches and 
bars. The daily flow and ebb tends to scour out bays and 
inlets, to break through the beach ridges, and to establish 
runways or channels leading across the foreland, marsh, 
or coastal plain. 

The Eastern Coast of the United States. — The coast of North Amer- 
ica from Yucatan to Nova Scotia exhibits, in great variety of detail, the 
results of the complex interaction of all the forces which shape the 
forms of the coast. The old Appalachian highlands have been worn 
down, and their debris has been spread over a belt 200 to 400 miles 
wide. Slow elevations and depressions have caused the shore line to 
swing back and forth across this belt repeatedly. Whatever position 
the shore line has occupied, the streams have always deposited the bulk 
of their sediment in the shallow water just off shore. Thus the zone 
of greatest deposition has swung back and forth with the shore line, 
and the sediment has been widely distributed. The result is a plain 
which slopes gently and evenly from the Piedmont plateau to the edge 
of the coast shelf. At the present time about half of this plain is above 
water and forms the Atlantic and Gulf coastal plain, which begins at 



236 



THE LAND 



New York and widens southward to Texas. The 
other half is under water and forms the surface of 
the continental or coast shelf. The present shore 
line south of Cape Cod is for the greater part of 
its length double. The inner or mainland coast 
is indented by numerous bays, those in the north 
being deeper than those in the south, the result of 
a moderate depression which has drowned the 
lower portions of all the river valleys. From one 
to ten or more miles off the points of the main- 
land, the outer shore line stretches in the long, 
smooth, swinging curves of a barrier beach. Be- 
tween the two lies a belt of lagoons, tidal 
marshes, and sounds. Along the Texas coast, 
tidal action is very feeble, and the beach is 
unbroken in one stretch of 102 miles. Off 
the great Mississippi delta the beaches are 
wanting or fragmentary. The form of the 
Florida coast is modified by the growth of 
coral reefs (Fig. 230). Along Georgia 
and South Carolina tidal action is strong, 
and the beach expands into the 
" sea islands," broken by many 
inlets (Fig. 229). The North Car 
lina coast is bordered by an 
almost continuous bar 
which extends in 
long curves 
from one cusp 
to another, 




fam 



Fig 227 



^/ |cs t„.i %± Cape Florida 

'.lief- ^ 




Fig. 228 



Fig. 230. 



2 3 8 



THE LAND 



Cape Cod 



CAPE 




NANTUCKET 
SOUND 



Fig. 231. 



the most prominent being Capes 
Fear, Lookout, and Hatteras. At 
the north end of New Jersey the 
beach skirts the foot of a sea cliff 
and projects far into the lower bay 
of New York, as the spit of Sandy 
Hook (Fig. 228). The southern 
shores of Long Island, Rhode 
Island, Marthas Vineyard, and 
Nantucket exhibit an almost con- 
tinuous line of lagoons and beaches. 
Cape Cod is an eroded headland 
(Fig. 231) which projects far into 
the sea, and from which long bars 
extend like wings to right and left. 
North of it, the characteristics of 
a glaciated coast become more 
prominent as the beaches disap- 
pear and are succeeded by the 
strongly contrasted coast of Maine, 



with its rocky islands, cliffs, and fiords (Figs. 227, 214). 

Realistic Exercise. — Students who live far inland, and have no 
opportunity to observe forms on the seacoast, may find most of the coast 
forms well developed along the shores of the Great Lakes. Even a 
small pond or temporary pool often presents in miniature the character- 
istic features of wave and current action. Perhaps the 
g^f^^ most favorable opportunity for the study 

of coast forms is furnished by the 




Fig. 232. Bonneville shore lines. 

shore lines of extinct lakes, like Bonneville (see p. 136), from which the 
water has retreated, leaving the bottom exposed to view. A dried-up 
pool beside the road will often repay careful study. 



CHAPTER XVIII 

THE PHYSIOGRAPHIC CYCLE AND THE CLASSIFICATION 
OF LAND FORMS 

The present form of the face of the earth is due to the 
action of two sets of forces. One set, derived from the 
internal heat of the earth itself, produces movements of de- 
formation in the earth-crust, or brings about a transference 
of matter from the interior to the exterior. The other set, 
derived from the heat of the sun, sets in play the various 
activities of the atmosphere, which. are chiefly outside the 
earth-crust. Gravitation is the constant ally and silent part- 
ner of all. Internal forces determine the larger features, 
such as ocean basins, continental blocks, mountain ridges, 
and volcanic domes. External forces modify these by pro- 
ducing out of them lesser features. One set rough-hews 
great blocks, which the other set shapes into forms of 
infinite detail. To one is due the apparently boundless 
expanse of oceans and continental plains which give an 
impression of vast sameness and monotony. To the other 
is due an equally vast variety. One produces an island 
like Hawaii (see Fig. 173), which the other transforms into 
an island like Santa Rosa (Fig. 233). The completed work 
of the one consists of profound deeps and lofty heights, 
areas of strongly contrasted elevation ; the completed work 
of the other would be the removal of this contrast, the 
lowering of elevations, the filling up of depressions, and 
the reduction of the face of the earth to a graded plain 
near sea level. Earth heat and sun heat work together, but 
at cross purposes : one to build up, the other to tear down. 

239 



240 



THE LAND 




Fig. 233. 

Yet each of these processes supplements the other. The 
waste of the mountains and plateaus goes to form beds of 
sediment which fill basins and valleys, build deltas, and 
bury coast shelves. Igneous granite and lava are con- 
verted into clay, sand, gravel, and dissolved salts ; these 
are consolidated into shale, sandstone, conglomerate, and 
limestone, and thus the total quantity of fragmental, aque- 
ous, and stratified rock is ever increasing. These beds ac- 
cumulate until they attain sufficient thickness, and are in 
turn upheaved to form again the massive strata of plateaus 
and mountains. Under the persistent stress of all these 
forces the materials of the earth-crust pass through a re- 
current cycle of forms of which the plateau or mountainous 
elevation and the graded plain near sea level are the ex- 
treme members. The regular succession of changes is 
often interrupted and the orderly procession of forms 
interfered with ; but every square mile of land surface is 
in some stage of development on the way from a plateau 



THE PHYSIOGRAPHIC CYCLE 



241 




•SpU13|SI DIUBOOQ 

•saSuBJ ucE^unoj/\j 

■sXa||BA 01L|doJlSBIQ 

■s>po|q peiijj. 

's;u9LudjB0S3 ■s;|n^j 

■sppj. ;snjL|;jaAo 

•sp|oj. passgjdujo^ 

•sppj ubj •sauipuA's 

'S8uipi;uy , s9U!pouo/\| 

■■e;bj;s pauipu 



•s? <?" A tf ^ j^ 

v *^ 

Ridges. Spurs. 
Mesas. Buttes. 
Knobs. Necks. 
Cliffs. Towers. 
Basins. Fiords. 
Plateau mountains. 
Relict mountains. 
Talus slopes. 
Alluvial plains. 
Lake plains. 
Marshes. Deltas. 
Filled valleys. 
Terraces. Bars. 
Mc 



ora '"es. Kames. 



V* 



Fig 234. — The physiographic cycle. 



242 



THE LAND 



to a graded plain, or from a graded plain to a plateau, and 
its features may be readily assigned to a definite place in 
an ideal order which may be called the physiographic cycle 
(Fig. 234). 

Classification. — The features of the land were formerly 
classified according to their outline, size, and superficial 
form, but since they have been more thoroughly studied 
many schemes have been proposed for their more scien- 
tific classification, according to structure and origin. No 
system of classification can be perfect ; many features are 
complex in structure and the products of more than one 
process. There will necessarily be some inconsistencies, 
and some features will fall into more than one group. 
The following scheme is based upon origin, the forms 
being placed in groups according to the agents and proc- 
esses which have produced them. Let the student fill out 
the groups indicated according to his own judgment, and 
suggest improvements if possible. 



I. Structural or Dystro- 
phic Forms, produced by deforma- 
tion or dislocation of the earth- 
crust. 

II. Accumulated Forms, pro- 
duced by the deposit or heaping up < 
of material. 



III. Sculptured Forms, pro- 
duced by the removal of material. 



By upheaval. 

By subsidence. 

By fracture. 

By folding. 

By running water. 

By ice. 

By wind. 

By waves, tides, and currents. 

By volcanism. 

By weathering. 

By running water. 

By ice. 

By wind. 

By waves, etc. 



BOOK III. THE SEA 

/ am the Sea I I hold the land 
As one holds an apple in his hand ; 
Hold it fast with sleepless eyes. 
Watching the continents sink and rise. 
Out of my bosom the mountains grow, 
Back to its depths they crumble slow. 
The iron cliffs that edge the land 
1 grind to pebbles and sift to sand ; 
I comfort the earth with rains and snows 
Till waves the harvest and laughs the rose. 
Flower and forest and child of breath 
With me have life — without me, death. 
The earth is a helpless child to me. 
I am the Sea ! 

— Charlotte Perkins Stetson. 

CHAPTER XIX 
THE FIGURE OF THE SEA 

The sea is an irregular, incomplete, spheroidal shell of 
water which covers about 72 per cent of the earth-crust, 
intervening between it and the atmosphere. The upper sur- 
face of the sea is convexly curved and comparatively smooth, 
while its lower surface conforms to the elevations and de- 
pressions of the earth-crust, and is irregular. Its thick- 
ness varies from zero to nearly six miles, averaging about 
two and one fifth miles. Compared with the size of the 
earth, the sea forms upon it only an insignificant film ; but 
the volume of the sea is nearly thirteen times that of the 
land above sea level. If all the land were shoveled into 
the Atlantic, it would fill only one third of that ocean. 

243 



244 THE SEA 

Oceanography, the science which treats of the sea, has come into 
existence only since the middle of the nineteenth century. 

While the general outline of the oceans has been known since the 
voyages of Cook (i 768-1 779), very little progress had then been made 
in the exploration of the sea bottom and the study of the properties and 
movements of sea water. Successful soundings in deep water were first 
made by Sir John Ross in 1840. He was also the first to dredge up 
material from the deep sea floor. 

Special apparatus designed for deep sea exploration has now reached 
a high degree of efficiency. Depths of the ocean are measured by 
means of a steel wire to which an iron tube is attached. The tube 
passes through a lead or iron weight hung upon a hook in such a man- 
ner that when the tube strikes the bottom the weight drops off. The 
tube is fitted with devices for bringing up specimens of water and mud 
from the bottom. At intervals along the wire, thermometers are at- 
tached which record the temperatures of the water. Dredges and nets 
of various kinds are dragged along the bottom to catch the animals 
which may be present. 

The greater part of our knowledge of the sea below the surface was 
obtained during the voyage of the Challenge?' (1872-1876), a ship fitted 
and sent out by the British government for the purpose of surveying 
the ocean basins. The Blake of the United States Coast Survey and 
vessels of other nations have made thorough explorations of limited 
portions of the sea, so that some areas of the sea bottom are now more 
accurately known than many areas of the land surface. 

The Surface of the Sea. — The "level of the sea," al- 
though not constant or uniform, is the standard from 
which all geographical heights and depths are measured. 

The centrifugal force of the earth's rotation tends to make the sur- 
face of the sea conform to that of a spheroid which is about thirteen 
miles farther from the center at the equator than at the poles ; but the 
constancy and uniformity of this spheroidal surface are disturbed by 
many causes : — 

(1) The attraction of the land masses tends to make the sea surface 
higher near all coasts, especially mountainous coasts, than in mid-ocean. 

(2) Ocean currents and on-shore winds tend to pile up the waters 
in deep bays in their course. 

(3) In regions of heavy rainfall, as in the equatorial rain belt, the 



THE FIGURE OF THE SEA 245 

surface of the sea probably stands higher than in regions where evapo- 
ration is rapid, as near the tropics. 

(4) Differences in atmospheric pressure in adjacent regions causes 
differences in sea level in those regions. Thus in the center of a 
tropical hurricane, the low pressure and inblowing winds heap up the 
water to a height which, when it occurs near shore, proves very destruc- 
tive as the water sweeps over the land. 

(5) Sedimentation is constantly filling up the ocean basins and 
tending to raise the sea level, while movements of , the earth-crust may 
change its level either up or down. 

(6) The winds cause waves, and the attraction of the sun and moon 
causes tides which disturb the surface of the sea. 

The Ocean Basins. — The sea is divided by the land 
masses into four basins of unequal size and widely vary- 
ing outline (see map, pp. 40, 41). 

The Basin of the Pacific Ocean comprises about 40 per 
cent of the whole sea area. It is roughly circular in out- 
line, with a diameter of about 10,000 miles. The main 
body of the Pacific Ocean has an average depth of two 
and three fourths miles. 

The Pacific is nearly closed at the north in latitude 65 , Bering Strait 
being both narrow and shallow. At the south it opens widely into the 
Southern Ocean. Its bed is traversed by numerous ridges which have 
a general northwest and southeast trend, and bear upon their tops 
thousands of small volcanic islands. It also contains several remark- 
able deeps. Fifty miles off the coast of Peru a sounding of 25,000 feet 
has been made, in the Tuscarora deep east of Japan one of 27,930 feet, 
in the Aldrich deep northeast of New Zealand one of 30,930 feet, and 
in the Challenger or Nero deep south of the Ladrone Islands, the deep- 
est sounding yet reported, 31,614 feet, or nearly six miles. The Pacific 
shore line on the American side is for the most part regular, and the 
slope from it abrupt. The northern and southern extremities are in- 
dented by numerous fiords. Along the northern and western sides of 
the ocean, peninsulas and lines of islands parallel with the coast inclose 
a continuous series of border seas. 

The Basin of the Atlantic Ocean comprises nearly one 
fourth of the sea area. Its length is 9000 miles, and its 



246 THE SEA 

least breadth between Africa and South America 
about 1700 miles. Together with the Arctic and 
Antarctic oceans it affords a broad channel of com- 
munication almost from pole to pole. The average 
depth of the main body of the Atlantic Ocean is 
about two and one half miles. 

The bottom is traversed lengthwise by an S-shaped central 
ridge over which the water is less than 12,000 feet deep. This 
ridge separates two valleys or troughs 15,000 to 20,000 feet 
deep, one extending along the American coast, and the other 
from the British Isles southward. At latitude 40 south the 
ridge ends, and the two valleys unite in a broad basin which 
penetrates far into the Antarctic regions and sinks on the 
polar circle to a depth of 25,000 feet. At the north the Atlantic 
basin is separated from the Arctic by a ridge which extends 
from the British Isles to Greenland and rises to a level less 
than 3000 feet below the surface. The greatest depth known 
in the Atlantic is 27.366 feet in the Blake deep near Porto 
Rico. Connected with the Atlantic are many inland seas of 
the largest size, the Mediterranean, Black, and Baltic on the 
east, and Hudson Bay and the Gulf of Mexico on the west; 
while the Caribbean and North seas belong to the class of 
border seas. 1 The coast shelf along the shores of the Atlan- 
tic is generally wide except on the African coast. 

The Basin of the Indian Ocean comprises about 
one eighth of the sea area and is in the shape of a 
triangle with a notch at its apex. Near the Afri- 
can coast it is broken by the large island of Mada- 
gascar, and its bottom is studded with numerous 
volcanic peaks, some of which rise above the sur- 
face. The main body of the Indian Ocean has an 
average depth of two and one fourth miles. 

The Basin of the Arctic Ocean comprises only 
about one thirtieth of the sea area and is nearly 
inclosed by the great northern land masses whose 

1 Inland seas are nearly inclosed by continents ; border seas by islands. 



THE FIGURE OF THE SEA 247 

shores surround it at about the seventieth parallel. Green- 
land projects northward beyond 8o°. An opening about 
1200 miles wide between Europe and America connects 
the Arctic with the Atlantic. The depth of the Arctic 
north of Eurasia was found by Nansen to be over 12,000 
feet. A great part of it is covered by drifting ice. 

The Southern Ocean is only a convenient term to desig- 
nate that part of the sea south of the fortieth parallel, from 
which the three great oceans diverge northward. It forms 
about one fifth of the sea area, and its average depth is 
more than two and one fourth miles. The region within 
the Antarctic circle is mostly occupied by a land mass or 
archipelago buried under an ice cap of great thickness 
(see p. 120). 

Origin. — The ocean basins not improbably owe their 
existence to differences in the original composition of the 
earth. According to the contraction theory (see p. 48), 
the earth has cooled and contracted more rapidly along 
some radial lines than along others. Those regions where 
radial contraction has been greatest are depressed. Ac- 
cording to the theory of isostasy, the earth-crust under the 
oceans is denser and heavier than under the continents, 
and has consequently gone down. Both theories imply 
that the ocean basins were formed at a very early period 
of the earth's history and have been permanent ever since 
— a conclusion which is sustained by considerable direct 
evidence (see p. 249). 

The Sea Floor. — The surface of the sea floor, being 
protected from the atmosphere, is free from all those 
features which are produced by erosion. Stream valleys, 
so common upon the land, are absent on the sea bottom 
except upon those portions of the coast shelf which were 
formerly exposed above sea level. Precipitous cliffs and 



248 



THE SEA 






i't 






pt^ 






Fig 236. — Continental 

deposit. 

(Magnified 10 times.) 



escarpments, probably due to faulting, have been discov- 
ered in some localities near the continents, but on the floor 
of the open sea generally the slopes are so gentle that a 
railroad could be run across it without 
grading or bridges. Of the structure of 
the sea floor, nothing is known except 
that volcanic cones seem to be more 
numerous than upon land. The floor is 
covered to an unknown depth with un- 
consolidated sediments of various kinds. 
Continental Deposits. — The waste of 
the land is brought down by streams and 
spread over the sea bottom by waves and 
currents. The coarsest sediment is deposited near shore, 
but the finest may be carried out several hundred miles. 
This material is easily recognized by the worn and rounded 
shape of its particles. Judging from the depth of similar 
deposits revealed by boringxon the coastal plains, it proba- 
bly attains near the land a thickness of several thousand 
feet. It covers about 14 per cent of the area of the sea 
bottom. 

Organic Deposits are being 
laid down everywhere on the 
sea floor. They consist of 
the shells and other hard 
parts of marine animals and 
plants. Every organism in 
the sea which has a shell 
or bony skeleton contributes 
something to this deposit, 
but the bulk of it is made up of the shells of minute ani- 
mals which live near the surface. As they die, their shells 
are continually falling through the water, like a gentle snow- 




Fig- 237 — Globigerina ooze 
(Magnified 13 times.) 



THE FIGURE OF THE SEA 249 

storm, and accumulate on the bottom as a soft, gray, Chalky 
ooze. Under the microscope this ooze may be divided 
into several varieties according to the preponderance of 
the shells of certain species or families, as globigerina 
ooze, pteropod ooze, diatom ooze, etc. Most of the shells 
are composed of carbonate of lime, but the diatom ooze 
consists of the siliceous cases of microscopic plants, which 
flourish in the cool and relatively fresh waters of the 
Southern Ocean. The lime deposits cover about 35 per 
cent, and the silica deposits about 7 per cent of the sea floor. 

Red Clay. — On the deepest parts of the sea floor the 
organic ooze disappears, and the surface 
is covered by a fine, sticky, red clay 
which becomes very hard when dry. 
Under the microscope it is found to con- 
tain sharp, angular grains of volcanic 
dust. It is also in part made up of the 
insoluble residue of organic deposits. 
As the shells settle through the water, Fig 238. - Red clay, 
they gradually dissolve until, all the < Ma § nified IO ° times -) 
lime disappears and only the insoluble matter remains. 

Along with the red clay occur great numbers of sharks' teeth which 
have resisted the solvent action of the sea water, and many irregular, 
rounded nodules of manganese of all sizes up to several inches in diam- 
eter. Many of the teeth belong to species which, are now extinct, and 
must have lain upon the sea bottom for untold ages without being 
covered up by the clay. The manganese lumps have been formed very 
slowly from the minute quantities of that element which exist in solu- 
tion in sea water, and are indications of the long period of time during 
which the depths of the sea have existed under conditions substantially 
the same as at present. The red clay covers about 35 per cent of the 
sea floor. 



DR. PHYS. GEOG. l6 




CHAPTER XX 

SEA WATER 

" We have in imagination been disposed to regard the waters of the 
sea as a great cushion placed between the air and the bottom of the 
ocean to defend and protect it from the abrading agencies of the atmos- 
phere." — Maury. 

Composition. — The ocean basins are filled with water 
which contains about 3| per cent of mineral matter in solu- 
tion. More than three fourths of this is common salt, proba- 
bly derived from the primitive atmosphere, which was hot 
enough to contain the water and salt in the form of vapor. 

The other minerals consist of salts of lime, magnesia, and potash, and 
minute quantities of almost every known element, all of which may 
have been brought into the sea from the land by rivers. The average 
composition of the salts of sea water is given in the following table : — 



Sodium chloride (common salt) 
Magnesium chloride 
Magnesium sulphate 
Calcium sulphate 
Potassium sulphate 
Magnesium bromide 
Calcium carbonate . 



77-758 
10.878 

4-737 
3.600 
2.465 
0.217 
°-345 



Sea water also contains the gases of the atmosphere dissolved in 
proportions which vary with the pressure and solubility of the gases, 
with the depth and temperature of the water, and with other conditions 
not fully understood. They are absorbed from the air at the surface 
and distributed through the depths by the movement of the water. 
They are subject to increase and decrease by the action of animal and 
plant life and by other chemical processes. The quantity of oxygen 
diminishes with increasing depth. The quantity of carbon dioxide in- 
creases as the temperature falls, and is greatest in the bottom waters. 

250 



SEA WATER 



251 



Temperature. — The rays of the sun do not penetrate 
the sea to a greater depth than about 600 feet. Conse- 
quently the deep water, comprising by far the greater part 
of the whole sea, has a constant temperature at any given 
spot throughout the year, while the surface water is sub- 
ject to seasonal and occasional changes of temperature. 

The map, p. 253, shows isothermal lines drawn through 
places of equal surface temperature. A mean annual 
surface temperature of 8o° or above is found in a belt 
about fifteen degrees wide lying on both sides of the 
equator in the Indian and western Pacific, and a much 
narrower belt lying north of the equator in the Atlantic 
and eastern Pacific. A mean annual surface temperature 
below 40 is found in the Arctic, the extreme northern 
Pacific, the Atlantic north of Newfoundland, Iceland, and 
Norway, and the Southern Ocean south of latitude 55 . 

The surface of the ocean may be divided into five great zones of 
temperature: (1) an intertropical zone with high temperature, 70 to 
90 , and a range or seasonal variation of less than io° ; (2 and 3) two 
circumpolar zones with a temperature below 40° and a range of less 
than io° ; (4 and 5) two intermediate zones with a range amounting to 
20 to 50 . The lowest recorded temperature of surface water in the 
open ocean is 29 near the Faroe Islands, and the highest 90 in the 
equatorial Pacific. The Red Sea and Persian Gulf sometimes reach 94 
and 96 . About 13 per cent of the sea surface has a temperature always 
below 40 , and about one half has a temperature always above 6o°. 

The temperature of the deep bottom water is everywhere 
low, varying from 29 in the polar regions to 35 under 
the equator. In the equatorial regions the temperature 
falls rapidly with increasing depth from the surface to 40 
at 2000 to 4800 feet, then slowly to the bottom. If we 
call the water above 40 warm, then the layer of warm 
water is nowhere more than 4800 feet thick, and in most 
places considerably less. Ninety-two per cent of the sea 



252 



THE SEA 



LAT. 

71' 



38 N. LAT. 23 N. 

F 74° F 



EQUATOR. LAT. 7 S. LAT. 20 S. LAT. 38 £ 

75° F 78° F 75' F 78° F 70° 05° 55° 54°F 



FEET 
1200 

2400 

3000 

4800 

6000 

7200 

8400 






















! 








: 
































! 


1 






: 






























— 


!___ 








: 












to 








_4>i_j 










; 








^r 










10 






















: 








: 








































; 
































; 








! 
































: 








35" 










35 


9600 
10800 
12000 


































! „ 








































y* 








: 
































| 








33'.S 








35.6° 
































! 




























34 




33 










! 




















35 '4 "35^, 










! 








! 

























Fig. 239— Section of the Atlantic Ocean, showing temperatures. 

floor is overlain by water having a temperature below 40 , 
while only three per cent of the floor has a temperature 
always above 6o°. Eighty per cent of all the water in the 
sea is below 40 , and the average temperature is between 
38 and 39 . 

The sun has been shining upon the sea for many millions of years, 
and, although its rays do not penetrate deeply, there has been time 
enough for the heat to reach the bottom by the slow process of conduc- 
tion. It might also be expected that the internal heat of the earth 
would do something toward keeping the lower waters warm. The 
fact that while the temperature of the earth-crust increases down- 
ward, the temperature of the sea decreases in the same direction, consti- 
tutes one of the most interesting problems of oceanic geography. The 
Mediterranean Sea is a large body of deep water (13,000 feet) shut off 
from the ocean by a barrier at the Strait of Gibraltar which rises to a 
level of 1200 feet below the surface. The temperature of the Mediter- 
ranean water falls from 75 at the surface to 55° at a depth of 750 feet, 
where it ceases to fall and remains constant at 55 all the way to the 
bottom. The temperature of the Atlantic water outside falls continu- 
ously from 75 at the surface to 37 at the bottom. The Red Sea has 
a temperature of 70 from 1200 feet to the bottom at 3600 feet. Here 
nature has contrived a suggestive experiment for us on a large scale by 



254 



THE SEA 



confining a body of deep water and showing that it can be kept warm to 
the bottom. It is also true of the Gulf of Mexico and other inclosed 
bodies of water, that their bottom temperature is about the same as that 
of the bottom of the deepest inlet from the ocean. The lesson plainly is 
that the low bottom temperatures of the open sea are due to a circu- 
lation which carries the cold polar waters along the bottom toward the 
equator, where they rise, become warmed, and evaporate or return 
toward the poles on the surface. The shallowness of the warm water 
at the equator is explained by the rise of cold water from the bottom. 
The escape of any considerable quantity of cold water from the Arctic 
basin is prevented by the shallowness of the openings, but the basins of 
the Atlantic, Pacific, and Indian may be regarded as great gulfs opening 
from the Southern Ocean, which acts as a refrigerator and controls their 
bottom temperatures, making them lower in the southern than in the 
northern part of each. The movement is not in the nature of a current, 

but a creep of the whole lower 
mass of water so slow that it 
does not stir the finest sedi- 
ment, and is discoverable only 
by its effects upon temperature. 
It is probably supplied by a 
sinking of the water at about 
50 south latitude, where the 
cold Antarctic and salt tropi- 
cal waters meet. Figure 240 

shows the effect upon tempera- 
Fig. 240. . . _ . , 

ture of a ridge in the ocean 

bottom. The northward movement of the water is so slow that it is 

stopped by the barrier, and the bottom temperature on the north side 

of it is no lower than the temperature at the level of its top. 

Average Temperature of the Sea at Various Depths 



40° 


39° 


38° 




-«= 






37° 




36° 




Uniformly 36° 

J 


\ 




35° 




34° 




33° 




32" 







Depth 


Temperature 


6oo feet 


60.7 


1,200 " 


50.O 


3,000 " 


40. 1 ° 


6,ooo " 


36-5 


13,200 " 


35-2° 



SEA WATER , 255 

Pressure and Density. — The pressure of a liquid is 
equal in all directions and proportional to its depth. The 
pressure of sea water one foot deep is 0.445 pound upon 
every square inch of surface. At the depth of a mile 
the pressure is 2350 pounds, or the pressure of sea water 
is more than one ton per square inch per mile of depth. 
The pressure at a depth of five or six miles would crush 
a hollow vessel of almost any material if it were not sus- 
tained by pressure within. 

Deep-sea thermometers have to be protected from pressure. A 
sealed glass tube containing air, lowered to a depth of 12,000 feet, 
was crushed to a fine powder. If water were easily compressible, 
its density at great depth would be greatly increased by the pressure 
of the water above. Water is but slightly compressible, so that 140 
cubic feet of surface water lowered to a depth of one mile would be 
compressed to 139 cubic feet, and at a depth of five miles its density 
would be increased 3! percent. The density of sea water varies also 
with the temperature, but depends chiefly upon the quantity of salts 
held in solution. It is greatest in the tropical regions of rapid evapora- 
tion, and least in the equatorial region of heavy rainfall and the polar 
regions of melting ice (see map, pp. 256, 257). 

Density of Water under Various Conditions 

Pure water at 39. 6° F. . . . . . . .1.00 

Surface sea water at 6o° F. . . . . . 1.024 to 1.03 

Sea water at a depth of five miles ..... 1.06 

Pure water at 212 F. . . . . . ; . .95 

Pure ice . . . . . . . . . .92 

Sea ice (with included air) .... about -9175 

On account of the low temperature and high pressure in the depths 
of the sea, the density of the water there must be considerably higher 
than that due to its saltness alone. To ascertain the actual density 
of the water, the density determined after the water has been raised to 
the surface must be corrected according to the temperature and pressure 
which exist at the depth from which it was taken. When this is done, 
it is found that the water lies in quite regular horizontal layers which 
increase in density downward. 



RELATIVE DENSITY 
OF THE SURFACE WATER,_ 




REFERENCE 
~| Specific Gravity less than 1 .025 
" 1.025 to 1.026 
» 1.026 to 1.027 
" 1.027 to 1.028 
" " more than 1.028 



'Warm Currents -^-> Cold Currents 



120 140 160 180 160 140 




ANTA *CTIC CIRCLE 



100 80 



256 






CHAPTER XXI 

MOVEMENTS OF THE SEA 

" No drop of the ocean, even at its greatest depth, is ever for one 
moment at rest." — Wharton. 

Waves may be produced by any disturbance of the sea 
surface, but they are usually the result of the friction of 
the wind. Waves are a series of parallel ridges and hol- 
lows which follow one another across the surface of the 
water. The parts of a wave are crest and trough; and 
the dimensions are length, of the distance from one crest 
to another, and Jieight, or vertical distance from the bottom 
of the trough to the top of the crest. Each wave appears 
to consist of a mass of water moving forward in the direc- 
tion of the wind, but, except in shallow places, the water 
moves forward and back and up and down. The wave 
moves forward by taking in continually new water in 
front and dropping out water behind. The motion may 
be imitated by shaking a sheet or strip of cloth up and 
down ; waves pass through it from end to end, but no 
portion of the cloth travels lengthwise. A flag blown 
out from the staff and waving in the wind performs 
similar vibrations. 

The movement of the water in a wave is not quite so simple as that 
of the cloth and is shown in Fig. 241. 

The line AB represents the surface of a water wave whose length 
is at, moving in the direction of the long arrow. Suppose the wave 
length to be divided into eight equal parts, ab, be, etc. If the water 
were still, the particles 1, 2, 3, etc., would lie directly above the points 
a, b, c, etc, but each particle is moving in a circular path in the direc- 
tion shown by the curved arrows. Particle 9 is at the lowest point, 8 

258 



MOVEMENTS OF THE SEA 259 

has moved one eighth of a circle farther than 9, 7 one eighth farther 
than 8, etc., 5 being half a revolution farther along than 9 and at the 
highest point. When the wave has advanced one half its length, each 
particle will have moved through one half a circle, 1 and 9 will be at the 
crest, and 5 at the bottom of the trough. Thus while the wave advances 
its whole length each particle of water makes a complete revolution. 
As shown by the small arrows, the column of water under the trough is 
moving backward, the column under the middle of the front slope up- 




- ^ v r- —^ 
f- I : 1 \ \ 4- 



a b c d e f g It i 
Fig. 241. 

ward, the column under the crest forward, and the column under the 
middle of the back slope downward ; but the distance moved diminishes 
with the depth as shown by the small circles. If the water were at 
rest, the dotted lines ai, &2, etc., would all be vertical and the columns 
of water inclosed by them would be of uniform width; but in the wave 
motion the lines sway back and forth, like the stalks of grain in a field 
when the wind blows, and the columns under the troughs become wider 
at the top, and the columns under the crests narrower. By the rise 
of the water the crest is continuously transferred to a point in front 
of its position at any given moment, and the trough is transferred in 
the same direction by the fall of the water. The path of a particle 
is not always circular, but varies with the form of the wave. It is often 
an ellipse with its long axis inclined or horizontal. 

The surface of the sea is never still. When the air is 
calm the influence of previous winds or of distant storms 
keeps up a long, low undulation called the ground swell. 
Storm waves sometimes reach a height of 50 feet and a 
length of 1 500 feet, and travel 60 miles an hour. As 
waves approach the shore and reach shallow water, there 
is not water enough to build up the front half to its full 



260 THE SEA 

dimensions, the front slope becomes steeper than the 
back slope, then perpendicular, and finally overhanging 
until the crest falls forward, forming a breaker. On shelv- 
ing shores the waves sometimes rise to a height of ioo 
feet or more, and the crests, containing many tons of 
water in rapid forward motion, strike blows which hardly 
anything can resist. Cliffs are pounded to pieces, light- 
houses destroyed, and breakwaters built of the heaviest 
masonry washed away. The pressure sometimes reaches 
2000 pounds per square foot. 




Fig. 242. — A breaker. 

Tides. — The level of the sea is subject to a regular, 
periodic rise and fall which is called the tide. It varies in 
amount at different places. On the deep, open ocean it is 
probably less than one foot. On the coasts of oceanic 
islands it is not more than six or seven feet, while at the 
heads of funnel-shaped inlets, like the Bay of Fundy, it 
amounts to as much as fifty feet. If we should watch the 
tide from any point along the coast at low water, we should 
see the rocks, bars, and portions of the beach and sea bottom 
laid bare ; then the water would slowly flow or creep up 
for several hours and cover them. High water would be 



MOVEMENTS OF THE SEA 



26l 





followed by an ebb or fall, lasting six hours or more. The 
interval between two periods of high water or low water is 

twelve hours and 
twenty-six minutes, 
but it is not always 
equally divided be- 
tween ebb and flow, 



Fig. 243. — Low tide. 

the rise being gen- 
erally more rapid 
than the fall. 

The difference of level 
between high and low g ' 244 ' ~~ lg * e ' 

water varies not only at different places, but at different times at the 
same place. These phenomena must have been observed by all peoples 
who have lived along the shore of the sea, and it must have been 
noticed at a very early period that the times of high and low water have 
some relation to the position and phases of the moon. The connection 
between the moon and the tides was not understood, however, until 
Newton's discovery of the law of gravitation. 

If the earth were a globe of water, it is easy to under- 
stand how the attraction of the moon would draw it out 
.of shape and produce a slight elongation or bulging in the 
direction of the moon. The effect upon the spheroidal 
shell of sea water is the same as though it were a complete 
sphere. 

Realistic Exercise. — Fill a toy balloon with water, but not too full, 
and fasten a cord to its neck. A gentle pull upon the cord will cause 
the sphere to become elongated. Swing the balloon around in a hori- 
zontal plane and the long axis will become horizontal. 

To produce this effect there must be two forces acting in opposite 
directions. We commonly think of the moon as revolving around the 



262 THE SEA 

earth, but the exact truth is that the earth and moon revolve together 
around their common center of gravity. If the earth and moon were 
connected by a rigid bar without weight, but strong enough to support 
both, the point in the bar where they would balance would be the center 
of gravity. Although the bar would be 240,000 miles long, the earth is 
so much heavier than the moon that the center of gravity falls within 
the mass of the earth about 3000 miles from the center. The earth and 
moon revolve around this point once in about twenty-eight days, if 
it were not for the centrifugal force generated by this revolution, the 
mutual attraction of the two would cause them to fall together, and their 
distance from each other is determined by the exact balancing of these 
two forces. At the center of the earth the balance is perfect, but on 
the side of the earth which is nearest to the moon attraction is a little 
stronger than at the center, and causes the water to bulge slightly 
toward the moon. On the side of the earth which is farthest away 
from the moon the attraction is a little weaker than at the center, and 
the unbalanced centrifugal force causes the water to bulge slightly away 
from the moon. In Fig. 245 G is the common center around which 
the earth and moon revolve. The arrows indicate by their length the 

L 








L 
Fig. 245. 

relative values of the two forces at the places in question. Thus there 
are at any given moment two places of high water, H, on opposite sides 
of the earth and two places of low water, Z, between them. 

If the moon were always above the same point on the 
earth, there would always be high water at that point, the 
moon would cause no change in the level of the sea any- 
where, and, consequently, there would be no lunar tides ; 



MOVEMENTS OF THE SEA 



263 



but, as the earth rotates on its axis from west to east, the 
point directly under the moon and the other points of high 
and low water travel around the earth from east to west at 
the same rate as the apparent motion of the moon. 

Thus every part of the sea has two stages of high water and two of 
low water within the time between two transits of the moon over any 
given place (24 hours and 52 minutes). The period is more than 
twenty-four hours, because the moon is actually moving in its orbit 
eastward in the same direction as the rotation of the earth, and after 
one rotation of the earth on its axis, it takes fifty-two minutes for any 
given point on the earth to overtake the moon. 



^FIRSTI QUARTER 




Fig. 246. 



The sun also produces tides in the sea in the same man- 
ner as the moon, but on account of its greater distance the 
solar tides are much smaller than the lunar. At new moon 
and full moon the sun, earth, and moon are all in the same 
straight line, as shown in Fig. 246, and the lunar and 
solar tides combine to produce a greater rise and fall than 
usual, called spring tide. At intermediate periods the sun 
and moon act at right angles to each other and produce 
a smaller rise and fall than usual, called neap tide. 

The variations in the moon's path and distance, the form and depth 
of the ocean basins, the outline of the continents, and the configuration 
of inlets produce endless variations in the height and period of the tides. 
The rise and fall are greatest at the heads of open-mouthed bays, where 
the crest of the tidal wave finds less and less room as it progresses 



264 



THE SEA 



toward the head, and least in landlocked seas like the Mediterranean 
and Gulf of Mexico. 

The tidal movement in the north Atlantic seems to be a slopping of 
the water back and forth as in a tilted basin, high water occurring on 
the west side at the same time as low water on the east. In straits con- 
necting two bodies of water which receive the tidal wave at independent 
mouths, as in East River between New York Bay and Long Island Sound, 
high water does not occur at both ends at the same time, and powerful 
and sometimes dangerous currents, called races, flow alternately in 




Fig. 247. — Bore, on the Seine River. 

opposite directions. Where the tide enters the mouth of a river, the 
water sometimes piles up into a wave with perpendicular front which 
travels upstream at high speed, washing away the banks and upsetting 
or filling boats. Such waves are called bores, and are especially nota- 
ble in the Amazon, Seine, and Severn rivers. 

Currents. — The surface waters of the sea are not only- 
subject to the to-and-fro movement of waves and tides, but 
also take part in a vast system of circulating currents by 
which the water is transferred to distant regions, tempera- 
ture and saltness are partly equalized, and the climate of 



MOVEMENTS OF THE SEA 265 

the land masses is greatly modified. The map on pp. 256, 
257 shows the location, direction, and extent of the princi- 
pal surface currents. The largest members of the system 
or trunk streams of each ocean basin extend parallel with 
the equator on each side of it. 

In the Pacific the North Equatorial current starts from the west coast 
of Mexico and flows westward about 8000 miles. Its width is 600 miles, 
its depth about 600 feet, and its velocity averages one mile per hour. 
The volume of water in motion is several hundred times greater than 
that carried by the largest river on land. It reaches the continental 
barrier at the Philippine Islands, where it divides, and the larger branch 
turns northward and eastward past the Japan Islands, where it is called 
the Kurosiwo or Japan current. In middle latitudes this current returns 
eastward across the north Pacific to the coast of North America. A 
large branch forms a reversed eddy along the coast of Alaska, but the 
greater part flows south, and, completing the circuit, rejoins the Equa- 
torial current. 

The South Equatorial current leaves the west coast of South America 
and flows westward 4000 miles without interruption. In mid-ocean it 
encounters the submarine ridges of the southwestern Pacific and sends 
numerous branches southward. A part of its water finally reaches Aus- 
tralia and flows south along the eastern coast. All these branches 
finally join the Antarctic drift, which moves eastward around the globe 
between 40 and 6o° south latitude, and some portion of the water, fol- 
lowing the coast of South America northward, completes the circuit. 
Between the two westward-flowing equatorial currents a small Counter 
current, made up of branches from either side, flows back eastward and 
completes an independent circuit. 

In the Atlantic Ocean the North Equatorial current leaves the coast 
of Africa and moves westward to the West Indies, where it is joined by 
a large branch from the South Equatorial current. A portion of the 
combined streams passes through the Caribbean Sea and Yucatan chan- 
nel, rounds the western end of Cuba, and emerges from Florida Strait as 
the Gulf Stream. A larger portion flows between and outside of the 
West India Islands and joins the Gulf Stream east of Florida and Geor- 
gia. The combined currents are here seventy-five miles wide with an 
average velocity of four miles per hour, and sweep the bottom at a 
depth of 2500 feet. The Gulf Stream follows the coast of the United 



266 THE SEA 

States as far as Cape Hatteras, where it leaves the land and crosses the 
ocean to the vicinity of the Azores Islands. Here it divides, a part 
returning southward and completing the circuit, while a greater part, 
spreading out over the north Atlantic, drifts slowly past the British 
Isles and Norway, far into the Arctic Ocean. This inflow of water into 
the Arctic is compensated by a strong outflow along the east coast of 
Greenland. This current rounds Cape Farewell, sends a large eddy 
northward to occupy Baffin Bay, and finally, under the name of the 
Labrador current, fills the space between the coast of North America 
and the Gulf Stream as far south as Chesapeake Bay, where it disap- 
pears by subsidence and mixture with the warmer water. 

In the south Atlantic the Equatorial current flows from the Gulf of 
Guinea westward to South America, where the sharp angle at Cape St. 
Roque splits it in two. The northern branch crosses the equator to 
join the northern circuit, but the southern branch follows the coast of 
South America nearly to its southern extremity. The water gradually 
turns eastward and, reenforced by the Antarctic drift, returns to the 
African coast and passes northward to the Equatorial, completing 
the circuit. The Equatorial Counter current in the Atlantic is 
pinched for want of room, but appears in the eastern part as the 
Guinea current. 

In the Indian Ocean south of the equator the circuit is similar to that 
of the other oceans, from Australia to Africa, southward to the Antarc- 
tic drift and back to Australia. 

The basin of the Indian Ocean north of the equator is small and 
obstructed by the peninsula of India. A circuit, however, exists, but 
with the peculiarity that in summer (May to October) the movement is 
in the regular direction, westward in mid-ocean and eastward along 
the Asiatic coast, and in winter (October to May) these directions are 
reversed. 

Generalization. — As a general statement, it may be said 
that the equatorial waters flow westward until they strike 
the edge of the continental block, where they turn pole- 
ward, recross the oceans in middle latitudes, and, returning 
toward the equator, complete the circuit. The principal 
movement of the water forms a great eddy on each side of 
the equator, turning in the northern hemisphere in the 
same direction as the hands of a clock (clockwise), in the 



MOVEMENTS OF THE SEA 



267 



southern hemisphere in the opposite direction (counter- 
clockwise). In the north Pacific the principal eddy throws 
off a smaller reverse eddy along the Alaskan coast. In 
the north Atlantic the Gulf Stream drift into the Arctic 
and the return by the Greenland-Labrador current form a 
reverse eddy as large and important as the primary one. 
In the Pacific and Atlantic Counter currents form small 
reverse eddies between the Equatorials. In the southern 
hemisphere the open water permits an Antarctic drift, 600 
to 1000 miles wide, to encircle the globe, into which the 
southern extremity of each continent projects and inter- 
cepts a portion of the stream. 



CURRENTS 

OFTHE 

GREAT LAKES 




i PENNSYLVANIA 
Cleveland 



Fig. 248. 



Cause of Currents. — It is well known that the wind blow- 
ing steadily for a considerable time is able to start surface 
currents in the same direction. This is demonstrated in 
the currents of the Great Lakes, where there are no differ- 



268 



THE SEA 



ences of temperature or saltness. It is also demonstrated 
by the currents of the Indian Ocean north of the equator, 
where the currents reverse their direction twice a year 
about one month after a similar change in the monsoon 
winds. It has been calculated that the force of the trade 
winds is sufficient in 100,000 years to set water in motion 
to the depth of 12,000 feet. If the map of the ocean cur- 
rents, pp. 256, 257, is compared with the map of the pre- 
vailing winds, p. 305, the correspondence is at once evident. 
The equatorial currents have the same position and direc- 



,V\\ \ 



f <w 








—*. —*. >^ 




C/vV 



Fig. 249. Fig. 250. 

Currents in pans of water, set up by blasts of air. 

tion as the trade winds, the eastward-flowing currents are 
in the same latitudes as the prevailing westerly winds, and 
the centers of the great oceanic eddies are coincident with 
the tropic calms, while the counter currents flow eastward 
in the belt of equatorial calms. The Gulf Stream - Green- 
land eddy follows the same course as the southwesterly 
and northeasterly winds which circulate around a center 
south of Greenland. The correspondence is so nearly 
complete as to warrant the conclusion that the principal 
cause of the surface currents is the prevailing winds. 

Realistic Exercise. — Sprinkle sawdust upon the surface of a pan 
or tank of water, and with a bellows, foot blower, or the mouth, blow 



MOVEMENTS OF THE SEA 269 

through a tube a current of air parallel with the surface of the water. 
At the place where the air current strikes the water there will be a slight 
depression of the surface, the water will flow in toward it from both 
sides, and a current will be set up which will move to the farther side 
of the pan, parallel with the blast of air, and returning will form a com- 
plete circuit on each side. 

An Englishman, Mr. Clayden, has made and exhibited a model of 
the Atlantic Ocean, over the water of which blasts of air are blown in 
the place and direction of the prevailing winds. The result is a repro- 
duction of the actual surface circulation, not only in its main features, 
but even in details like the turn of the Greenland current around Cape 
Farewell and the Baffin Bay eddy. 

It has been maintained that differences of temperature 
and saltness might be causes of ocean currents, and it is 
probable that they do exercise some influence upon the 
movement, but it is impossible to determine just what that 
influence is. They play a subordinate part, and their ef- 
fects are masked and overcome by other forces. As accu- 
rate knowledge of the sea increases, it becomes more and 
more evident that the happy guess made by Benjamin 
Franklin more than a century ago is the true explanation, 
and that nearly every detail of the circulation of surface 
waters in the sea may be satisfactorily accounted for by 
the force of the winds and the effect of land barriers. 

Effects of Ocean Currents. — Surface currents carry their 
own temperature into the regions which they penetrate, 
and, by imparting it to the air above, modify the climate of 
neighboring land masses over which this air moves. Be- 
tween the tropics the western part of each ocean is flooded 
with water which has made a long journey under a nearly 
vertical sun and has become heated to a high temperature ; 
hence the belt of water above yo° F. is widest and deepest 
along the eastern coasts of the continents. The east sides 
of the oceans in the same latitudes are supplied with water 



270 THE SEA 

by currents coming from higher latitudes and also by 
water which rises from below in place of the water blown 
away by the trade winds ; hence the west coasts of the con- 
tinents are washed by water of comparatively low temper- 
ature. This contrast is plainly seen on the opposite sides 
of Africa and South America (see map, p. 253). 

In middle and high latitudes of the northern hemisphere, 
west coasts are washed by warm currents and east coasts 
by cold currents. 

The most extreme case of this kind is presented by the north Atlan- 
tic, where on the east side the isotherms are carried far northward by the 
Gulf Stream, and on the west side far southward by the Labrador cur- 
rent (see map, p. 253). Thus the ocean currents cooperate with the 
prevailing winds to make northwestern Europe habitable and to keep 
its harbors free from ice almost to the Arctic Circle, while Greenland 
and Labrador in the same latitude are ice-bound, snow-covered, and 
desolate. The same contrast exists in a less degree on opposite sides 
of the north Pacific. 

Sea Ice. — The freezing point of sea water varies with 
its saltness from 32 to 26 . In the Arctic Ocean the 
ice forms every winter to a thickness of ten or fifteen 
feet. Thzfloe ox pack thus formed is broken up by tides 
and storms, and driven about by winds and currents. 
The pieces are forced against one another and the shore, 
and piled up in extreme confusion. The surface becomes 
so rough that it is almost impossible to travel over it. 
Lieutenant Markham's party north of Greenland was able 
to accomplish only seventy miles in forty days. Nansen's 
ship, the Fram, was able to make way through the ice by 
blasting it with dynamite. The largest masses are called 
flocbergs. The shores of Greenland are crowded with 
both floebergs and icebergs (Fig. 86) derived from the 
glacial tongues of the ice cap (see p. 120). Both the sea 
and the land ice are carried far southward by the Labra- 



THE SEA AND MAN 271 

dor current, and the largest bergs finally melt in the Gulf 
Stream. In the winter of 1 872-1 873 a part of the crew of 
the ship Polaris drifted on an ice floe from the north of 
Baffin Bay to the coast of southern Labrador, nearly 2000 
miles, in six and a half months. 

A still larger quantity of ice is supplied by the Antarctic 
ice cap, from which bergs 200 to 500 feet above water 
and sometimes several miles in length are continually be- 
ing discharged. The Antarctic bergs are flat-topped and 
steep-sided (Fig. 8y), and exhibit a stratified structure of 
alternate blue and white layers, which probably represent 
the snow-fall of successive summers and winters. As only 

i 1 1 -i An ,- ^ 



I B# 



Fig. 251. 

about one tenth of the mass of ice stands above water, the 
total thickness of these bergs must be from 2000 to 5000 
feet. 

As the form of an iceberg changes by irregular melting, 
it may turn over several times, always assuming a position 
in which its center of gravity is as high as possible. A 
berg having the form A (Fig. 251), unless weighted at the 
bottom, would take the position B, and C would turn upside 
down like D. Icebergs often transport considerable quan- 
tities of stones and dirt which have fallen from shore cliffs 
or have been dragged from the land. As the ice melts, 
these are distributed over the sea bottom far from their 
place of origin. 

The Sea and Man. — The sea is the original source of 
the moisture in the air. The vapor, borne by the winds 

DR. PHYS. GEOG. — 1 7 



272 THE SEA 

over the land, falls as rain, supplying water for the use 
of plants and animals, and the flow of streams. The most 
productive lands are those which are near the sea or are 
accessible by winds from the sea. 

The sea teems with living forms, many species of which, 
like the codfish, mackerel, herring, and oyster, form staple 
articles of food. The whale furnishes oil and the fur seal 
furnishes fur, both of such value that these animals have 
been nearly exterminated. From the sea are obtained 
sponges, corals, and pearls of great commercial value. 

Large numbers of people seek the sea for health and 
pleasure. A sea voyage is a favorite method of travel, 
and the most popular resorts are those which afford sea 
air and sea bathing. 

The sea was once regarded with dread and terror as 
being dangerous and destructive. It is often thought of as 
a barren, unproductive "waste of waters." To many peo- 
ples it has been an impassable barrier to migration. At 
first men crept timidly in small boats along the shore ; but 
gradually they gained courage to venture out of sight of 
land and, guided by the stars, to find their way across the 
waters to distant countries. The most progressive peoples 
now use the sea as a means of communication and trade. 
Large vessels traverse it in all directions, carrying the 
products of every land to every other land. Civilized man 
has changed the sea from a barrier to a broad, easy high- 
way of commerce. Many of the great cities of the world 
are great because they are seaports. The most prosperous 
and enlightened countries have a long seacoast. Russia 
loses no opportunity to secure ports upon the Pacific and 
Atlantic, and Great Britain has gained her high place 
among nations through her control of the sea. 



BOOK IV. THE ATMOSPHERE 

CHAPTER XXII 

THE AIR 

Composition. — The atmosphere, or gaseous portion of 
the earth, forms a complete spheroidal shell which sur- 
rounds the solid and liquid globe, and not only rests upon 
the surface of land and sea, but also penetrates them to a 
great depth. Its thickness, which is not definitely known, 
is certainly several hundred miles and may be many thou- 
sand. Its bulk is almost entirely made up of five gases, 
which are present in the proportions given in the following 

table : — 

Composition of the Air 



Per cent 
of Volume 



Density 



Nitrogen 

Oxygen 

Water vapor (average) . 

Argon 

Carbon dioxide (average) 
Air 



76.95 

20.61 

1.40 

1. 00 

0.03 



99.99 



.971 

1. 105 

.624 

1.380 
1.529 
1. 000 



These gases are not united or combined in any way, but 
are almost entirely independent of one another. They act 
like five separate and distinct atmospheres occupying the 
same space at the same time. The space which each gas 
occupies is determined by the balance between its own 

273 



274 THE ATMOSPHERE 

expansive force, tending to make it expand indefinitely, 
and gravitation, which holds it down to the earth. Car- 
bon dioxide, being the heaviest of all these gases, does 
not extend so far upward as the others. Oxygen is a 
little heavier than nitrogen, and its relative proportion 
decreases slightly in the upper air. Water vapor is the 
lightest of all, but its existence as vapor is so far depend- 
ent upon a warm temperature that it is almost absent at 
great heights. 

Properties and Functions. — Oxygen combines freely 
with nearly all the elements, and in its numerous com- 
pounds forms about one half of the whole weight of the 
globe. By the process of respiration it supports the life 
of all plants and animals, and it is the universal agent of 
combustion. By respiration, combustion, decay, and other 
processes of oxidation the quantity of oxygen in the air is 
being continually diminished. This loss is partly compen- 
sated by the oxygen set free from plants in the process of 
food manufacture. 

Nitrogen is extremely inert and enters into combination 
with other elements with difficulty. To it is due nearly 
three fourths of the pressure and density of the air. 
Without it birds could not fly, clouds and smoke would 
settle to the ground, and the force of the wind would be 
proportionately diminished. 

Argon resembles nitrogen, with which it was confounded 
until near the end of the nineteenth century. 

Carbon dioxide (C0 2 ), or carbonic acid gas, is a com- 
pound of carbon and oxygen formed in the active growing 
parts of plants and in the tissues of all animals and given 
off by them in the process of respiration. It is also pro- 
duced by the combustion of all the ordinary forms of fuel, 
and sometimes escapes in large quantities from active vol- 



THE AIR 



275 



canoes, old volcanic regions, and from many mineral 
springs. It forms the chief food supply of plants. The 
green parts of plants in the sunlight absorb carbon dioxide, 
separate it into its elements, retain the carbon, and give off 
the oxygen. Carbon dioxide plays an active part in rock 
formation, entering into combination with lime and other 
bases to form limestones. It also enters largely into the 
composition of the bones and shells of animals. While 
the absolute quantity of carbon dioxide is the least of 
all the principal constituents of the air, the part it plays 
in the economy of nature is second to none. 

Water vapor is supplied by evaporation 
from all damp surfaces, but chiefly from the 
sea. When cooled it condenses again into 
water and falls as rain and snow. The quan- 
tity present in the air at different times and 
places is very variable, amounting sometimes 
to three per cent. 

Visibility of Air. — The air is sometimes visible. 
When thrown into agitation by heat it may be seen 
rising from a stove or from the heated ground. Under 
proper conditions of illumination and background the 
wind may be seen as plainly as a current of water. 

Weight and Pressure. — At sea level a cubic 
foot of air weighs about one ounce and a 
quarter, and the weight of all the air above 
sea level produces an average pressure of 
14.74 pounds upon every square inch of sur- 
face. This pressure is equal in all directions, 
— downwards, upwards, or sidewise at any 
angle. The pressure of the air is measured 
by the barometer. 

If a glass tube about 32 inches long is filled with 
mercury, inverted, and the open end inserted into a cup 



y 



Fig. 252. — Simple 
forms of barom- 
eters. 



276 THE ATMOSPHERE 

of mercury, the mercury in the tube will fall until only enough remains 
to balance the weight of a column of air of the same size extending to 
the top of the atmosphere. Such an arrangement is essentially a mer- 
curial barometer. We can not weigh the column of air directly, but 
we can weigh the column of mercury which balances it. The height of 
such a column at sea level averages about 30 inches, and, if one square 
inch in area of cross section, weighs 14.74 pounds. If the barometer is 
carried to higher elevations, there will be less air above it, and the mer- 
cury will fall. If the pressure of the air increases, it will drive more 
mercury into the tube. The pressure of the air is measured and ex- 
pressed in terms of the height of the column of mercury which it sup- 
ports. When the air pressure is said to be 29.50 inches, it means that 
the air pressure is sufficient to support a column of mercury 29.50 inches 
high. A description of the standard barometer and instructions for its 
use are given in the Appendix, pp. 400, 401. 

Density. — The air being easily compressed, its density 
is proportional to the pressure to which it is subjected, 
and consequently diminishes as the height above the sea 
increases. Density and pressure are also influenced by 
other conditions, of which temperature and humidity, or 
quantity of water vapor it contains, are the most impor- 
tant. When air is heated it expands and becomes less 
dense. The same effect is produced by the addition of 
water vapor. On warm, damp days the pressure and 
density are less, and the barometer stands lower than on 
cold, dry days. 

Temperature. — The temperature of the air is deter- 
mined by the amount of heat received and absorbed from 
the sun and earth. As the sun heat passes through the 
air on its way to the earth, about one third of it is absorbed 
by the air and goes to raise its temperature, while the re- 
maining two thirds reaches the surface of the land and 
water. A part of this is reflected back without warming 
the earth and another part, being absorbed, goes to raise 
or maintain the temperature of the land and water. The 



THE AIR 



277 




Barometer 
in Inches. 



Density of 
air. 



Height in 




F A 



■■ " - )■ ' 



HIMALAYA MOUNTAINS 



Fig. 253. — Decrease of density and amount of air with increase of altitude. 

warm earth in turn warms the air next to it slightly by 
conduction and still more by radiating its heat upward. 

Of the heat reflected and radiated from the earth about 60 per cent 
is absorbed by the air and goes to raise its temperature still further. 
The heated air radiates some of its heat back to the earth, and so a 
continual exchange of heat is going on between the air and the earth ; 
but on the whole and in the long run as much heat escapes from the 
earth as it receives. The air takes toll as the heat passes through it 
both ways, coming and going, and temporarily retains about 70 per 
cent of the whole amount supplied from the sun'. 

T/ie lower air absorbs much more heat than the upper 
air, and consequently is maintained at a higher tempera- 
ture. This is due largely to the presence of cloud, fog, 
dust, and smoke, which may be regarded as atmospheric 



278 



THE ATMOSPHERE 



sediment held in suspension somewhat as fine mud is 
suspended in water. The larger proportions of carbon 
dioxide and water vapor in the lower air also increase its 
absorptive power for heat. 

If the air were perfectly clear, dry, and free from carbon dioxide, the 
heat of the sun would reach the earth with slight obstruction, and in 
the daytime the land would become excessively heated. In the night 
the heat would escape with equal rapidity, and the land would become 
excessively cooled. Upon lofty mountains which reach up through the 
zone of dust, cloud, and water vapor this is the actual condition. The 
clouds act as a blanket to protect the earth from the fierce heat of the 
sun by day and to prevent the escape of heat at night, thus maintaining 
a more equable temperature. 




Fig. 254. 

In Fig. 254 the horizontal line at the bottom represents the surface 
of the land or water, and the dotted line indicates an elevation of ten 
miles. The total heat received from the sun is represented by ten 
arrows, of which three are stopped by the air, and seven reach the 
earth. The seven arrows pointing upward represent the heat given 
off from the earth, of which four are stopped by the air and three pass 
directly through into space. 

The temperature of the air diminishes on an average one degree for 
every 300 feet of elevation. The average temperature of the air also 
diminishes from the equator to the poles at the rate of about one degree 
for every degree of latitude ; this is due chiefly to the spheroidal form 
of the earth, which causes the sun's rays to strike more obliquely and to 
be distributed over more space toward the poles (see p. 22). The dis- 
tribution of temperature in the atmosphere is subject to these two gen- 
eral laws, but is made quite irregular by various influences, which will 
be discussed later. 



THE AIR 279 

The Measurement of Temperature. — Temperature is 
measured by the thermometer, several varieties of which 
are described in the Appendix (pp. 398, 399). 

Weather Observations. — Every student should provide himself with 
the best thermometer he can afford. Its error may be determined by 
comparison with a standard thermometer in the laboratory. Compari- 
sons should be made at two or more temperatures, one at or below 
freezing, and one near ioo°. The thermometer should be placed in 
a suitable position at home. The north side of a post at some distance 
from any building, and four feet from the ground, will answer the 
purpose. At two periods every day, morning and evening, as between 
7 and 8 a.m., and between 7 and 8 p.m., let the student read and 
record the temperature, observing and recording at the same time the 
direction of the wind as shown by a vane placed above trees and 
buildings ; the state of the sky as to clearness or cloudiness ; the fall 
of rain or snow, and any other notable phenomenon of the weather, 
as fog, hail, frost, etc. These observations should be continued for at 
least three months, and, if possible, for a year. The record may be 
kept in the following form : — 

Wind 
Date Hour Temp. _ Remarks 

and Sky 

The direction of the wind may be indicated by an arrow flying with the 
wind as on a weather map, the state of the sky by an open or shaded 
circle, attached to the arrow ; O means a northwest wind with clear 
sky ; < Q an east wind with sky half cloudy ; >f a south wind with 
sky overcast. * 



CHAPTER XXIII 
MOISTURE IN THE AIR 

Evaporation. — Under suitable conditions, evaporation 
takes place from all damp surfaces and the water vapor 
mingles with the surrounding air. The heat in the water 
makes the molecules vibrate and some of those at the sur- 
face fly off into space. Ice evaporates as well as water, but 
the higher the temperature, the more rapid is the evapora- 
tion, and at boiling point molecules escape from all parts 
of the water, forming bubbles of' steam. At the moment 
of evaporation, water expands to about 1 700 times its liquid 
volume and is transformed into an invisible gas or vapor. 
The quantity of vapor which can exist in any given space de- 
pends upon the temperature of the vapor. When the space 
contains all it can hold, the vapor is said to be saturated. 

Grains of Saturated Water Vapor in a Cubic Foot at 
Various Temperatures 



IO° 


•776 


34° 


2.279 


58 


5-37o 


82 


I I.626 


12° 


.856 


36° 


2 


457 


6o° 


5-745 


84 


I2.356 


14° 


.941 


38 


2 


646 


62 


6.142 


86° 


13.127 


1 6° 


1.032 


40° 


2 


849 


64° 


6.563 


88° 


13-937 


1 8° 


1. 128 


42° 


3 


064 


66° 


7.009 


90° 


14.790 


20° 


1-235 


44° 


3 


294 


68° 


7.480 


92° 


1*5.689 


22° 


J-355 


46° 


3 


539 


70 


7.980 


94° 


16.634 


24° 


1.483 


48 


3 


800 


72° 


8.508 


96 


17.626 


26° 


1.623 


50° 


4 


076 


74° 


9.066 


98" 


18.671 


28° 


1-773 


52° 


4 


372 


76° 


9-655 


IOO° 


19.766 


3°° 


i-935 


54° 


4 


685 


78 


10.277 


I02° 


20.917 


32° 


2. 113 


56° 


5 


016 


8o° 


10.934 


104° 


22.125 



280 



MOISTURE IN THE AIR 281 

The quantities given in the table on p. 280 are the same whether the 
space is a vacuum or is already filled with air or other gases. The air 
has nothing to do with evaporation except to retard it. Water evapo- 
rates more rapidly into an absolutely empty space than into dry air, but 
at a given temperature the same quantity will evaporate into each. When 
water vapor is added to air, the expansive power of the mixture is in- 
creased, the surrounding drier air is pushed away, and the whole mass of 
moist air expands until its density becomes less than that of the drier air. 
If a cubic foot of dry air at a temperature of 8o° and weighing 516 
grains rests upon a body of water and evaporation takes place until 
1 1 grains of water vapor is added, the whole mass of the mixture will 
weigh 527 grains, but a cubic foot of it will weigh only 510 grains, and 
will be less dense than the original dry air. 

Humidity. — The quantity of water vapor actually present 
in space or air is called its absolute humidity. The quan- 
tity which the space might contain if full or saturated is 
called its capacity. The ratio of the absolute humidity to 
the capacity is called the relative humidity. 

If an eight-ounce bottle contains two ounces of water, its absolute 
humidity may be said to be two ounces^ its capacity eight ounces, and 
its relative humidity two eighths or 25 per cent. 

Absolute humidity answers the question, how much is there in it? 
capacity answers the question, how much will it hold ? relative humidity 
answers the question, how full is it? If the relative humidity of air is 
above 80 per cent, it may be said to be damp air ; if below 50 per cent, 
dry air. Whether air is dry or damp depends not only upon the quan- 
tity of moisture which it contains, but also upon its capacity as deter- 
mined by temperature. Air at 32 containing two grains of water 
vapor to the cubic foot is very damp because nearly saturated. If heated 
to 70 , ij; would still contain two grains, but would be very dry because 
only one fourth saturated. On the other hand, dry air may become 
damp by cooling without the addition of any moisture. 

Realistic Exercise. — Fill a bright tin cup half full of water at the 
temperature of the room, add a few lumps of ice, and stir the mixture 
with a thermometer. Watch carefully the outer surface of the cup and 
at the moment it becomes dulled by the formation of dew, read the 
thermometer. The vapor in contact with the cup has been cooled to 
saturation and has begun to condense. The temperature at which this 



282 THE ATMOSPHERE 

occurs is called the dew-point. Since vapor at the dew-point is satu- 
rated, absolute humidity at dew-point equals capacity. By reference to 
the table on p. 280 the absolute humidity may be found. Suppose the 
dew-point to be 40 , then the absolute humidity or actual quantity of 
vapor present is 2.849 grains per cubic foot. If the temperature of the 
room is jo°, its capacity according to the table is 7.980 grains per cubic 
foot, and its relative humidity is 2.849 "*" 7-9%°i or 35-7 P er cent. 

Hygrometer. — A more convenient method of measuring relative 
humidity is by means of the hygrometer (see Appendix, pp. 401, 402). 

Condensation. — When water vapor is cooled below the 
point of saturation, condensation takes place and the 
vapor changes to fog, cloud, rain, snow, hail, dew, or frost. 
Cooling in the atmosphere is brought about by several 
processes. 

(1) Expansion. — Wherever air rises and reaches successive levels 
of less pressure, it expands and some of its heat energy is expended 
in pushing away the surrounding air. Thus without any transfer of 
heat to other bodies, it is cooled simply by its own expansion one de- 
gree for every 183 feet of ascent. 

This is called mechanical cooling and is one of the most efficient 
causes of condensation. Air at yo° F. and of 50 per cent relative 
humidity would become saturated by a rise of 4000 feet. As soon as 
condensation begins, the latent heat of the water vapor is liberated and 
the cooling by expansion is retarded. Descending air is warmed by 
compression one degree for every 183 feet of descent. 

(2) Radiation. — Air is cooled by radiating its heat to cooler objects 
in the vicinity, as the ground, the sea, a mass of ice or snow, or a body 
of cooler air. This cause is most efficient in currents of air moving in 
any direction from warmer to cooler regions. 

(3) Conduction. — When air comes in actual contact with a cooler 
body, it loses some of its heat by conduction. This process is com- 
paratively unimportant because air is a poor conductor of heat, and only 
a thin layer of it next to the cooler body is affected. 

(4) Mixture. — Air is often cooled by mixture with cooler air. This 
is not a different and distinct process, but furnishes favorable conditions 
for rapid radiation and conduction. 

Clouds. — The condensation of water vapor in the air 
near the earth produces fog ; at higher altitudes, cloud. 



MOISTURE IN THE AIR 283 

Clouds are composed of minute particles of liquid water or 
of ice, a sort of water dust. Unless borne up by a rising 
current, they settle slowly through the air, but on reaching 
a stratum of unsaturated air again evaporate. Generally 
condensation continues above and thus the cloud persists, 
although continually destroyed and renewed. When vapor 
is carried horizontally forward by an air current, it may 
condense at one place and evaporate farther on : thus the 
cloud appears to be moving 
forward, but does not ex- 
tend beyond a certain point. 




This process is strikingly shown 
by the " banner cloud " which 
sometimes hangs for hours from _. 

a mountain peak, like a nag at- 
tached to a staff. A current of saturated air chilled by the mountain 
condenses on the leeward side and evaporates at some distance beyond. 
This cloud is a temporary form assumed by the vapor as it passes 
through a certain space. The ever changing forms of clouds are largely 
_, ._ . due to evaporation and renewal. 

Cloud Forms. — All the numer- 
ous cloud forms may be classed 
under four principal types : — 
(1) Cumulus clouds are 
■i: :% rounded masses like heaps of 
wool, generally formed at the top 
of an ascending column of air. 
Their horizontal base marks the 
level where saturation is reached, 
and above this condensation may 
continue until the cloud is piled 
up to the height of five miles or 

more. Cumulus clouds are char- 
Fig. 256. — Cumulus. 

acteristic of the equatorial re- 
gions and of warm summer afternoons elsewhere when the columns 
of air started upward by the heat of the sun have reached a considerable 
height. They often result in showers and thunderstorms. 




284 



THE ATMOSPHERE 




Fig. 257. — Cirrus 

(3) Stratus clouds extend in 
long, horizontal bands or layers 
and vary in height from 1000 
feet to three miles. 

(4) Nimbus clouds are those 
from which snow or rain is fall- 
ing. They may be formed from 
stratus or cumulus clouds. 

Many combinations and in- 
termediate forms occur, of which 
cirro-stratus, cirro-cumulus, and 



(2) Cirrus clouds 
are light and feathery, 
resembling ostrich 
plumes, dabs of thin 
white paint, loose 
wisps of straw, a cat's 
tail, and various fan- 
tastic forms. They are 
formed at heights of 
five or more miles and 
consist of minute ice 
crystals or snowflakes. 





Fig- 259. —Nimbus. 



strato-cumulus 
are the most 
common. 

Precipita- 
tion. When 
water vapor 
condenses 
into parti- 
cles so large 
that the air 



MOISTURE IN THE AIR 285 

can no longer support or evaporate them, a falling or pre- 
cipitation occurs in the form of rain, snow, or hail. As 
the particles fall through saturated air, condensation con- 
tinues, and the drops grow larger up to a certain limit of 
size, when they break into smaller drops. If the condensa- 
tion occurs at a temperature below freezing, the vapor 
crystallizes into snowflakes. Of these there are numerous 
forms, but all agree in having angles of 6o° between their 
branches and in being six-pointed or six-sided. Snow or 
rain may evaporate before reaching the earth. 




Fig. 260. -Snow crystals, magnified. 

Hailstones are masses of ice, or of ice and snow, which are some- 
times as large as hens' eggs or even larger. Their structure is often 
complicated by alternate layers of snow and ice, showing that they have 
passed through a variety of atmospheric conditions. The exact method 
of their formation is not well understood. 

Measurement of Precipitation. — Rainfall is caught in a 
metal cylinder called a rain gauge, and its depth is meas- 
ured in inches (see Appendix, p. 402). Snowfall is de- 
termined by melting the snow in the gauge and measuring 
the depth of water produced. On an average ten inches 
of snow makes one inch of water, but the proportion is 
very variable. 

Dew and Frost. — When the temperature of any surface 
falls below the dew-point of the surrounding air, water 



286 THE ATMOSPHERE 

vapor begins to condense upon it in the form of dew. If 
the temperature of the surface is below freezing, the vapor 
crystallizes directly into hoarfrost. 

The dew does not fall, but is formed at the place where it appears. 
Frost is not frozen dew any more than snow is frozen rain. Much 
of the vapor which goes to form dew escapes from the ground or is 
given off from the surface of growing plants. Dew is heavier on a clear 
night because the heat is then radiated from the earth more rapidly than 
on a cloudy night. A tree, board, piece of paper, or cover of any kind 
acts like cloud and may keep the air beneath from cooling to dew-point. 
The under side of a board or stone next to the ground may be covered 
with a heavy dew while the upper side remains dry. This is due to the 
rise of vapor from the ground. Dew is heavier upon grass than upon 
bare ground, because of the excess of vapor given off from the grass, 
and because grass is a better radiator than earth. Dew is heavier in 
a valley than upon a hill top, because there is more moisture in the 
ground there to evaporate, and because the cooler and heavier air 
settles down into the valleys and lifts the warmer air out. A breeze 
prevents the formation of dew by keeping the air near the ground 
stirred up and mixed with the drier and warmer air above. The con- 
ditions favorable or unfavorable for the formation of dew and frost are 
often very delicately adjusted, and a slight difference in elevation, ex- 
posure, or condition of the surface will determine whether dew or frost 
will occur or not. 



CHAPTER XXIV 

WINDS 

Atmospheric Convection. — When air is heated or made 
more damp by addition of water vapor, it expands and 
becomes less dense than the surrounding air, which crowds 
in from all sides and buoys the lighter air upward. The 
draught in a stove or lamp is a familiar example of the 
rise of light air, and the smoke from a fire burning in 
the open air shows that there is an upward current in 
this, case also. Careful observation will discover the 
movement of the cool air toward the fire. If there is 
no wind stirring, the rising column of smoke may be seen 
to spread out horizontally when it reaches a stratum of 
air of its own density. A downward movement at some 
distance from the fire takes place, but is too slow to be 
easily detected. 

Every wind that blows is a part of some convection 
circuit, which may be hundreds or thousands of miles in 
extent. The air is set in motion and the movement is 
kept up by a difference in the atmospheric pressures over 
different parts of the earth's surface. The upward and 
downward currents of the circuit are usually beyond the 
reach of ordinary observation, and in the regions where 
they leave or reach the surface of the earth the air is 
apparently calm. The lower horizontal currents consti- 
tute the commonly observed winds, but somewhere in the 
upper air there is always a current in a nearly opposite 
direction, which is sometimes made perceptible by the 
movement of clouds. 

DR. PHYS. GEOG. — l8 287 



THE ATMOSPHERE 



Pressure and Winds. — The direction and force of the 
winds are always dependent upon differences of atmos- 
pheric pressure and can be explained as the result of the 
distribution of pressure. The location of regions of high 
and low pressure are shown on a map by the use of 
isobars or lines drawn through places of equal pressure. 

Figure 261 shows the isobars of the eastern part of the United States 
for the morning of March 26, 1898. The figures attached to each line 
show the height of the barometer along that line. The highest pres- 
sure was 30.8 inches in 
New England and the low- 
est 29.9 inches in Wiscon- 
sin, the difference being 
0.9 inches. If a barome- 
ter could have been carried 
very rapidly westward along 
the line AB, it would have 
fallen at the rate of one 
tenth of an inch for every 
100 miles. The rate of 
change of pressure along 
any line crossing the iso- 
bars is called the baro- 
metric gradient or pressure 
slope. It is evident that the 
rate of change of pressure 
is greatest, or, in other 
words, the slope is steepest, 
along a line which crosses 
the isobars at right angles, 
and that where the isobars 
are close together the slope 
is steeper than where they 




Fig. 261. 

Isobars. 



Isotherms.) 



are far apart. Gravitation tends to make the air move by its own weight 
down the steepest slope from high to low pressure. The arrows on the 
map fly with the wind and show that the wind was moving down the 
slope along lines parallel to CB, and not in the direction of the steepest 
slope AB. The cause of this slant in the direction of air movement 



WINDS 



289 



will be explained on pp. 
290-292. In Fig. 262 the 
directions of slope AB and 
ofthewind^/ Care opposite 
to those in Fig. 261. The 
isobars are farther apart, 
showing that the slope is 
less steep (one tenth of an 
inch to 150 miles), and the 
wind has a smaller velocity. 

These examples il- 
lustrate the first law 
of winds : By the force 
of gravitation winds 
always blow from a 
region of high pres- 
sure to a region of low 
pressure with a veloc- 
ity which varies with 
the steepness of the 
pressure slope. 




Fig. 262. 



Velocity of the Wind. — The average wind velocities in the United 
States vary at different localities between four and fourteen miles per hour. 
The velocity is greater over the sea than over the land and increases 
very rapidly with altitude for a few hundred feet and then more slowly. 
Velocities of 200 miles per hour have been observed at high altitudes 
and in tornadoes on the surface of the land. (See Appendix, p. 404.) 

Effect of Rotation of the 
Earth. — If a person is 
walking upon the deck of 
a moving steamer toward 
some fixed object upon the 
shore ahead of the ship, and the pilot turns the steamer to 
the left while the walker continues in the same direction as 
before, his course will be deflected toward the right-hand 



Fig. 263. 



290 



THE ATMOSPHERE 



side of the steamer and he will describe upon the deck a 
curved path leading to the right-hand side. The same 
effect is noticed in walking through a car while it is 
running around a curve. The walker tends to move 
straight on and is thrown against the seats on one side 
of the car. The rotation of the earth tends to produce a 
similar effect upon all moving bodies. If a globe is viewed 
from a point directly above the north pole while it is rotated 
from west to east, the northern hemisphere will be seen 
turning counterclockwise about the pole as a center. Every 
north-south line is constantly changing its direction in 
space. The same is true of any portion of an east-west line. 
These facts are shown upon the map, Fig. 264. Each meridian, as 
A, is carried by the rotation of the earth to new positions B, C, D, etc. 

If an arrow starting northward 
on A continues in the same di- 
rection, when carried around to 
C it will be moving northeast- 
ward. An arrow starting west- 
ward on E, when carried to H 
will be moving northwestward. 
An arrow starting southward 
on /, when carried to K will 
be moving southwestward. An 
arrow starting eastward on Mi 
when carried to P will be mov- 
ing southeastward. At all points 
on a rotating earth, except at 
the equator, directions are con- 
tinually changing so that if any 
moving body could maintain absolutely its original direction it would 
move toward all points of the compass in the course of one rotation. 
On account of friction no moving body can maintain absolutely its 
original direction, yet it is deflected by rotation more or less rapidly 
with a force which increases from the equator to the poles. The deflec- 
tion is to the right in the northern hemisphere and to the left in the 
southern. This is known as FerreVs Law. 




Fig. 264. 



WINDS 



291 



M 




Fig. 265. 



N 



Figure 265 shows the path of a body starting northward from any 
point O in the northern hemisphere and moving without friction. As 
it reaches higher latitudes, the deflection 
is more rapid and it turns to the east and 
south. As it returns toward the equator 
the deflection is less rapid toward the 
west, and it would thus describe a series of 
loops around the earth toward the west, 
between the parallels M and N. The 
form and limits of the loops would vary with the latitude and the speed. 
Realistic Exercise. — The subject of deflection by the rotation of the 
earth is a difficult one to explain and to understand, and has been erro- 
neously stated in many text-books. The deflection is not a lagging 
behind or running ahead due to increasing or lessening speed of rota- 
tion. Perhaps the simplest illus- 
tration of the deflective effect of 
rotation may be made as follows : 
On a sheet of pasteboard draw two 
straight lines crossing at the center 
at right angles, and mark the ends of 

£ the lines north, south, east, and west. 

Lay the sheet on the table in such a 
position that the south-to-north line 
extends from the observer toward 
some fixed object beyond. Start a 
pencil along the line toward the 
north, and while the sheet is rotated 
counterclockwise, keep the pencil 
moving toward the fixed object. The line made by the moving pencil 
will curve away from the straight line to the right or to the east of 
north on the sheet. If the pencil is started east along the west-to-east 
line, the result will be a curve to the south of east. The pencil does 
not change its direction in space, but as the sheet rotates under it, its 
direction on the sheet continually changes, and. always to the right of 
its course at any given moment, as in the northern hemisphere. If 
the sheet is rotated clockwise, the lines will curve to the left, as in the 
southern hemisphere. The more rapid the motion of the pencil, the 
less sharp will be the curve ; the more rapid the rotation, the more 
sharp the curve. A similar illustration may be made with chalk on 



W— 



S 
Fig. 266. 



292 THE ATMOSPHERE 

a black globe. An open umbrella mounted so as to turn upon its stick 
as an axis answers the purpose of a black globe. 

The winds are probably deflected more than any other 
moving body by the earth's rotation, and from this arises 
the second law of the winds : On account of the earttis rota- 
tion the path of the winds down a pressure slope in the 
northern hemisphere is to the right of a line perpendicular 
to the isobars, and in the southern hemisphere to the left. 
(See Figs. 261, 262.) The angle at which the wind 
crosses the isobars increases with the latitude. 



CHAPTER XXV 

INSOLATION AND TEMPERATURE 

The Distribution of Insolation. — The distribution of tem- 
perature, of pressure, of winds, and of rainfall over the 
face of the earth are so closely related that they can not 
be understood separately. They all depend primarily upon 
the distribution of the rays of the sun, or insolation, and 
this is determined chiefly by the form, attitude, and 
motions of the earth, as explained in Chapter I. On 
account of the spheroidal form of the earth there is but 
one ray of the sun that strikes its surface vertically, and 
the amount of insolation received decreases in every direc- 
tion from the point which receives the vertical ray. 

At the equinoxes, March 21 and September 23, the vertical ray strikes 
the equator, but at the winter solstice, December 22, the vertical ray 
strikes the Tropic of Capricorn, and at the summer solstice, June 21, it 
strikes the Tropic of Cancer. Thus the tropics bound a zone of maxi- 
mum insolation which receives the vertical ray of the sun during some 
portion of the year. At the equinoxes the tangent rays reach to either 
pole, but at the solstices they strike the polar circles, 231 beyond one 
pole and 23J short of the other. Thus the polar circles bound areas 
of minimum insolation which receive the tangent rays of the sun at 
noon during some portion of the year. Between the tropics and polar 
circles are zones of medium insolation. The belts of equal insolation 
on any given day are bounded by parallels of latitude, but they swing 
back and forth, north and south, through the year, following the appar- 
ent daily path of the sun through the heavens. 

During the year the sun shines an equal number of hours upon all 
parts of the earth's surface, but in the polar regions the insolation is 
nearly all received in the summer, while near the equator it is almost 
equally distributed through the year. 

The general result is shown in Fig. 270 (at the left), which gives the 

293 



294 THE ATMOSPHERE 

total insolation received in a year at different latitudes, expressed in 
percentages of that received at the equator. All places within the 
tropics receive more than 90 per cent as much insolation as the equator, 
while all places within the polar circles receive less than 50 per cent, 
the amount at the poles being about 42 per cent. 

The Distribution of Temperature is shown upon a map by 
means of isotherms or lines of equal temperature. Figure 
267 shows the mean annual temperature, and Figs. 268, 
269, the mean temperatures of January and July, each 
being the coldest month in one hemisphere and the warm- 
est in the other. 

The isotherms are based upon millions of temperature observations 
made in all parts of the world. For the annual isotherms the average 
of the temperatures for all the days of the year in each area of one or 
two square degrees is calculated ; for the monthly isotherms the average 
of all the temperatures recorded for the month. In these maps the 
effect of elevation is eliminated and all temperatures are reduced to 
what they would be at sea level, by adding a definite amount to the ob- 
served temperature. The observed mean annual temperature of Salt 
Lake City is 5 1 .3°, its elevation above the sea is 4300 feet, 8.7 is added 
and the annual isotherm of 6o° is drawn through it. Isotherms show- 
ing the actual temperature as affected by elevation would be too crooked 
and irregular to be shown upon a map of this scale. On the ordinary 
weather and temperature maps of a limited area, like the United States, 
the isotherms show the actual surface temperatures. 

It is evident from the maps that the distribution of tem- 
perature is quite different from that of insolation. While 
the isotherms extend in a general east-west direction (why ?) 
they are irregular in course and spacing. The irregularity 
is greater in the northern hemisphere than in the southern, 
and in January than in July. 

Effect of Land and Water. — If the surface of the earth 
were all water or all land of uniform elevation, the iso- 
therms would be parallels of latitude and the temperature 
would decrease regularly and equally along each meridian 
from the equator to the poles. In the winter the isotherms 



296 THE ATMOSPHERE 

bend toward the equator over the land, and away from the 
equator over the water, showing that the land is colder than 
the water. In the summer these conditions are reversed. 

At least five causes combine to produce this result. 

(1) Difference in Capacity for Heat. — It requires about twice as 
much heat to raise the temperature of a cubic foot of water one degree 
as it does to raise the temperature' of an equal bulk of land. Hence 
from this cause alone a land surface receiving the same amount of insola- 
tion as an equal water surface is warmed twice as rapidly. 

(2) Difference in Penetrability for Heat. — The sun's rays can not 
penetrate the land deeply, and as the land is a poor conductor of heat 
only a thin layer is warmed, while the rays penetrate the water to the 
depth of 600 feet, and the heat is distributed through a much larger 
volume of water than of land. 

(3) Difference in Mobility. — The land is fixed while the water is 
movable. The water which is warmed often flows away and its place is 
taken by cooler water. Thus the heat received by the land is concen- 
trated and that received by the water is diffused. 

(4) Difference in Evaporation. — About one half the heat received by 
the water is expended in producing evaporation and does not raise the 
temperature of the water. 

(5) Difference in Cloudiness. — Cloud and fog are more prevalent 
over the water than over the land, and these retard the heat on its 
way to and from the water. 

In spring and summer the land is heated more rapidly 
than the water, and in autumn and winter, being a better 
radiator and having less heat to lose, it cools more rapidly. 
The rise and fall of temperature in the water is less than 
on the land and is retarded in time, reaching a maximum 
in the northern hemisphere in August, and a minimum in 
February. The temperature of a land surface is subject to 
great variation, that of a water surface to small variation. 

Effect of Winds and Currents. — The irregularity of the iso- 
therms is partly due to ocean currents and prevailing winds 
which carry their own temperature into regions which other- 
wise would be warmer or colder. The northward bend of 



INSOLATION AND TEMPERATURE 297 

the isotherms on the west coasts of Africa and South Amer- 
ica is due to the cold ocean currents from the Antarctic 
drift, and the southward bend on the east coasts is due to 
the warm equatorial currents. The great bend of the iso- 
therms northward on the European side of the north Atlan- 
tic is due to the Gulf Stream and the southwest winds 
which accompany it, and the southward bend on the 
American side is due to the Labrador current. 

Regions of Maximum and Minimum Temperature. — In January the 
regions of lowest temperature are in northeastern Asia and Greenland 
(why ?), and the regions of highest temperature in Australia and South 
Africa (why ?). In July the areas of low temperature are in the vicinity 
of the poles, and the areas of high temperature in North Africa, south- 
western Asia, and southwestern North America (why ?). The lowest 
temperature ever recorded is — 96° in northeastern America, the high- 
est shade temperature 1 54 in the Sahara. A line drawn through the 
points of highest temperature on each meridian is called the thermal 
equator. It swings north and south with the sun, but is much farther 
from the geographical equator in July than in January (why ?). 

Zones of Temperature. — The tropics and polar circles do 
not divide the face of the earth into zones of temperature, 
but of insolation. The true temperature zones are bounded 
by isotherms. Any division of zones must be somewhat 
arbitrary, but the isotherms which mark the monthly aver- 
ages of 30 and yo° are convenient boundaries. The tor- 
rid zone lies between the isotherms of 70 on each side of 
the equator, the temperate zones between those of Jo° and 
30 , and each frigid zone is inclosed by that of 30 . 

If the position of each of these zones in January and July is observed 
on the maps, it will be seen that they all swing north and south with 
the sun. The change of position is greater in the northern hemisphere 
than in the southern. The torrid zone shifts about fifteen degrees in 
latitude and is widest in July, especially over Asia. In January the 
north temperate zone is narrow and irregular, but in July it widens so 
far as to crowd the north frigid zone out of existence. 



298 

INSOLA 
437. 



THE ATMOSPHERE 




Fig. 270. — Temperature zones. 

(Insolation in percentages at left.) 

By drawing the isotherms of Jo° and 30 for January and July upon 
one map, as in Fig. 270, we obtain a set of zones which are not shifting 
but fixed, and reveal in a striking manner the temperature conditions 
of the globe. Upon this map hot means an average temperature in the 
hottest month above jo°, cold an average temperature in the coldest 
month below 30 , and temperate an average monthly temperature 
between 70 and 30 . The space between the tropics is mostly occu- 
pied by a belt which is always hot, a truly torrid zone. This is bordered 
upon either side by a belt in which the summers are hot and the winters 
temperate. In the southern hemisphere there is a truly temperate zone 
in which both summer and winter are temperate. In the northern 
hemisphere this temperate zone is confined to the oceans. Over the 
land masses it is replaced by regions of exactly opposite conditions, 
a truly intemperate zone, in which the summers are hot and the winters 
cold. Beyond the temperate zones are belts of cold winters and tem- 
perate summers, and in the southern hemisphere only there is, around 
the pole, a truly frigid zone which is always cold. 

Range of Temperature. — The difference between the 
lowest and the highest temperature at any given place is 
called the range. It may be reckoned between the high- 



300 



THE ATMOSPHERE 



est and lowest temperatures observed in twenty-four hours, 
which gives the daily range ; between the average tempera- 
tures of July and January, which gives the annual range ; 
between the absolutely highest temperature observed dur- 
ing the year and the absolutely lowest, which gives the 
absolute annual range; and in other ways. 

The map, p. 299, shows that the average annual range increases 
with the latitude (why ?), and with distance from the sea (why ?), and 
is greater in the northern hemisphere than in the southern (why ?). The 
centers of maximum range nearly coincide with those of minimum tem- 
perature. The greatest absolute range is 182° at Verkhoyansk, Siberia. 

Figures 271 and 272 show the influence of latitude and of land and 
water upon range of temperature. 



J. 


F. 


M. 


A. 


M. 


J. 


J. 


A. 


s. 


0. 


N. 


D. 


86° 


J. 


F. 


M. 


A. 


M. 


J. 


J. 


A. 


s. 


0. 


N. 


D. 






































"53" 












_B_ 














B 








B 






































A 




















N, 




M 






































50° 
32° 

14 
-4° 


M 






















jvr 


A 
























Bd 












V 






































V . 




















_F_ 












FT, 










v^_ 
















































,8'P 


























V s 


P/ 












































































































































































































































\n 
























FC 


-40° 




























FC 




















































NFL 


JEN 


CE 


)F 


ATI 


ruD 


~~ 










INF 


.UE 


ICE 


OF 


LAN 


D A 


JD 


SEA 







Fig. 271. 



Fig. 272. 



Annual variation of temperature. 



B, 


Batavia 


latitude 


6° 8'S 


M, 


Madeira 


latitude 32°38' N 


A, 


Algiers 


" 


3 6° 47 ' N 


Bd, 


Bagdad 


" 33°2o' N 


P, 


Paris 


" 


48°5o* N 


V, 


Valentia 


" Si°SS' N 


SP, 


St. Petersburg 


" 


59° 5 6' N 


N, 


Nerchinsk 


" 5i°58' N 


FC, 


Fort Conger 


" 


8i°44' N 









CHAPTER XXVI 

THE DISTRIBUTION OF PRESSURE AND WINDS 

The Distribution of Pressure. — Isobaric maps may show 
the distribution of the average pressure for the year or 
month, or the actual pressure existing at a given day and 
hour. As on isothermal maps, the effect of altitude is 
eliminated, and all pressures are reduced to sea level by 
adding to the observed pressure the pressure of a column 
of air extending from sea level up to the height of the 
place of observation. The quantity to be added to the 
reading of the barometer varies with the temperature and 
density of the air at the time and place of observation, 
and furnishes a problem of unusual difficulty. The aver- 
age addition is about one tenth of an inch for every 100 
feet of elevation. (See p. 407.) 

Figure 274 shows that the regions of high average annual pressure 
(above 30 inches) form two nearly continuous belts around the globe, 
situated near 30 south latitude and 40 north latitude. The northern 
belt is the more irregular and is widest over the land. In the equatorial 
and polar regions the pressure is low, or below 30 inches, the lowest so 
far as known being at about 6o° south latitude. 

Figure 275 shows that in January the northern belt of high pressure 
is expanded so that it covers the greater part of the land surface, but 
is interrupted by a large area of low pressure over the north Atlantic 
and Arctic oceans and Greenland and a smaller one over the north 
Pacific. The southern belt of high pressure is broken up into three 
centers which lie over the oceans. The highest pressure, 30.50 inches, 
is found in central Asia; the lowest in the northern hemisphere, 29.50 
inches, near Iceland ; and the lowest of all, 29 inches, in the Antarctic 
regions. 

Figure 276 shows that in July the northern belt of high pressure shrinks 

301 



302 



THE ATMOSPHERE 



to two centers situated over the oceans, and that central Asia is occu- 
pied by a large area of low pressure, tailing at the center to 29.40 
inches. The southern belt of High pressure is nearly continuous along 
the tropic, with centers of higher pressure over the oceans and Australia. 

In general, the pressure rises from about 29.90 inches 
at the equator to a maximum of from 30.10 to 30.20 inches 
at 30 south latitude and 40 north latitude, then falls 
toward either pole. Toward the south pole the fall is 
regular and very rapid ; toward the north pole it is intef- 



N 70 60 50 40 30 



10 20 30 40 













































































































































.*>.,. 


































i2_ 


W: 























t-Ot 


JO 


ly\. 
















^ 












v~ 


















C. 


JAN. 
















\ 
























































A 














/& 










































i 


c - 


V 










w c 












































'c 






















































% 

































































Fig. 273. —Variation of atmospheric pressure along prime meridian in January, 
and 40 W. Longitude in July. 

rupted by local depressions. Figure 273 shows the varia- 
tion of pressure along the meridian of o° in January and 
of 40 west longitude in July. 

Relations of Temperature and Pressure. — A comparison 
of the isobaric and isothermal maps reveals the funda- 
mental relations between pressure and temperature. The 
persistently high temperature in the equatorial belt is ac- 
companied by persistently low pressure. In the seasonal 
changes the low temperature over the land in winter is 
accompanied by high pressure, the high temperature in 
summer by low pressure. These changes and contrasts 
in both temperature and pressure are more marked in the 
northern hemisphere than in the southern, and are extreme 
over Asia, the largest land mass. In January the low tern- 



304 THE ATMOSPHERE 

peratures which prevail over the northern hemisphere 
are accompanied by prevailing high pressure, and the cen- 
ters of low pressure occur over the warmer oceans. In 
July the universal high temperature in the northern hemi- 
sphere is accompanied by almost equally widespread low 
pressure. The centers of high pressure occur over the 
cooler oceans. These correspondences are in accordance 
with the well-known law that the density of air varies in- 
versely with the temperature. 

The fall of pressure with the fall of temperature from middle latitudes 
toward the poles, and the extremely low pressures in high southern 
latitudes, are apparent contradictions to this law and must be due to 
some other cause which overcomes the effect of low temperature. This 
subject will be considered later in connection with the winds (p. 311). 

The Relations of Pressure and Winds. — The direction and 
force of the prevailing winds have been determined by 
millions of observations in all parts of the world, but are 
best known over the oceans from the reports of sailors 
and naval officers. They are shown upon the wind maps, 
Figs. 277, 278, and the isobaric maps, pp. 303, 308, 309, 
by arrows which fly with the wind. The intimate relation 
which exists between wind movement and the distribution 
of pressure is clearly evident upon the isobaric map for 
January. The strong centers of high pressure in the 
southern oceans are each surrounded by a mass of air which 
is moving spirally outward, counterclockwise. The strong 
centers of low pressure in the northern oceans are each 
surrounded by a mass of air which is moving spirally in- 
ward, counterclockwise. In July the strong centers of 
high pressure in the northern oceans are each surrounded 
by a mass of air which is moving spirally outward, clock- 
wise. All centers of high and low pressure are accom- 
panied by similar movements, more or less regular and ex- 



ocean winds. January and February 




Less_ than 13 Miles an hour —^ Varjab[e mds 

j. Steady ii 

Fig 277. 

OCEAN WINDS. JULY AND AUCUST 




Less than 18 Miles an hour \ VarlabJe Wlnds 



Over 



Fig. 278. 
305 



^ St&ac/y n 



306 THE ATMOSPHERE 

tensive. Figure 279 shows that these movements are in ac- 
cordance with the laws of the winds given on pp. 289, 292 and 
are the results of gravitation and the rotation of the earth. 



NORTHERN SOUTHERN NORTHERN SOUTHERN 




CYCLONE ANTICYCLONE 

Fig. 279. 

Gravitation tends to make air move out from a center of high pres- 
sure down the steepest pressure slope, that is, along radial lines. The 
earth's rotation deflects the moving air to the right of the radial line in 
the northern hemisphere and to the left in the southern. Gravitation 
tends to make air move in toward a center of low pressure along radial 
lines. The earth's rotation deflects the moving air to the right of the 
radial line in the northern hemisphere and to the left in the southern, 
but can never make it move up the pressure slope against gravity. The 
result, in the northern hemisphere, is a curve in the form of the figure 6. 
As the wind approaches the center, its path becomes more nearly paral- 
lel with the isobars, and a whirl or eddy is set up. A movement of air 
spirally inward toward a center of low pressure is called a cyclone. A 
movement of air spirally outward from a center of high pressure is called 
an anticyclone. Near the center of a cyclone the air moves spirally up- 
ward ; near the center of an anticyclone there is a downward movement. 

Wind Belts. — The arrangement of the centers of high 
pressure in belts on each side of the equator causes the 
prevailing winds also to be arranged in more or less 
definite belts around the earth. The air moves from the 
belts of high pressure toward the equator on each side and 
is deflected westward by the rotation of the earth. This 
constitutes the trade winds, from the northeast in the 
northern hemisphere, and from the southeast in the south- 
ern. They blow with great steadiness throughout the 
year and are called constant winds. The air also moves 



THE DISTRIBUTION OF PRESSURE AND WINDS 



307 



from the belts of high pressure toward each pole and is 
deflected eastward. This constitutes the antitrade winds 
or prevailing westerlies, from the southwest in the north- 
ern hemisphere and from the northwest in the southern. 
This movement is most regular and forcible in the southern 
hemisphere, where the pressure slope is steep and constant. 
Between these belts of prevailing winds are the belt of 
equatorial calms, where the air is rising, and the belts of 
tropical calms, where the air is 
descending. The complete ideal 
scheme is shown in Fig. 280. 

Monsoons. — The belts of pressure 
and prevailing winds are not absolutely 
fixed, but swing north and south with 
the sun. The equatorial calm belt coin- 
cides not with the geographical equator, 
but with the thermal equator, and is 
most variable in position. The belts of 
the southern hemisphere are nearly con- 
stant. The widest departure from the 
ideal system is brought about by the large and elevated land mass 
of Asia. In summer this region ceases to be a part of the northern 
belt of high pressure, and becomes virtually a part of the equatorial belt 
of low pressure. Consequently the regular northeast trade winds are 
suspended. The southeast trades cross the equator and continue far 
northward as south and southeast winds over the western Pacific Ocean 
and eastern Asia, and southwest winds over the Indian Ocean and 
southern Asia. In winter the regular northeast trades prevail over 
these regions. These southerly summer winds and northerly winter 
winds are called monsoons. 

The Polar Whirls. — The air moving from the belts of 
high pressure toward the poles is deflected eastward, and 
acquires a high velocity. It thus forms, especially in the 
southern hemisphere, a great cyclonic whirl from west 
to east around the polar regions, rising gradually as it 
nears the center (see maps, pp. 308, 309). 




Fig. 280. 




3°9 



3io 



THE ATMOSPHERE 



There is some evidence to show that at the very center of the south 
polar whirl there is a small anticyclone, from which southerly winds 
blow outward with great violence. 

In the northern hemisphere the polar whirl is much interrupted by 
the land masses, and in summer it almost disappears. In winter it is 
divided into two portions which circulate around the centers of low 
pressure over the north Atlantic and north Pacific oceans. Probably 
a portion of the air moves in a circuit which incloses both centers. 

The General Circulation of the Atmosphere. — Thus far 
only the movements in the lower layers of air next to the 
surface of the land and water have been considered. 

Our knowledge of the upper air by direct observation is much less 
extensive and accurate than of the lower air. It has been gained by 
means of observations made upon mountains, by occasional balloon 

ascensions, from the drift 
of high clouds and vol- 
canic dust, and by means 
of kites and unmanned 
balloons carrying self-re- 
cording instruments, 
such as the thermograph 
and barograph (see Ap- 
pendix, pp. 402, 403), to 
great heights. 

The movement of 
the upper air is 
everywhere toward 
the poles, with an 
eastward deflection. 
In other words, the 
polar whirls in the 
upper air cover the 
whole of each hemi- 




Fig. 281. — Direction of primary air currents. 

(After Ferrel.) 



sphere and have a common circumference at the equator. 
As the currents approach the poles, they descend and re- 
turn at intermediate heights toward the equator. Below 



THE DISTRIBUTION OF PRESSURE AND WINDS 311 




this system of circulation, and fed by it, are the surface 
currents with which we are most familiar. The whole sys- 
tem of atmospheric circulation is shown by map and sec- 
tion in Fig. 281. 

On the map or shaded part of the figure, the complete arrows show 
the direction of surface currents, and the broken arrows that of upper 
currents. The mass of warm air rising from 
the equatorial regions, at and above a height 
of two miles, turns to the northeast and south- 
east. By a very circuitous, spiral course, pass- 
ing round the earth many times on the way, 
but moving with increasing velocity, it ap- 
proaches the poles and gradually descending 
returns toward the equator. At the tropical 
belts of high pressure, the return currents 
drop down to the surface of the earth and 
continue as the trade winds to the equator. A part of them turn back 
at the tropics and form the prevailing surface westerly winds or anti- 
trades, which rise as they approach the poles and rejoin the intermediate 
return currents. Figure 282 shows a diagram of the upper currents 
and a simplified section of the whole system. 

Effect of the Polar Whirls. — The effect of the polar whirls may be 
seen in the rapid rotation of water in a pan or bowl. The centrifugal 
force throws the water away from the center, where the surface becomes 
depressed, and piles it up around the sides, where the surface becomes 
elevated, as in Fig. 283. The water being deeper' at A and B than at 
C, its pressure upon the bottom is proportionately 
greater. A similar effect is produced by the whirl 
of the air around the polar regions. It is thrown 
away from the polar regions and piled up around 
the circumference of the whirl. There is less air 
above the polar regions than above latitude 3o°-40°, and the atmos- 
pheric pressure is correspondingly low at one place and high at the 
other. Thus the centrifugal force of the polar whirl makes the pressure 
low in spite of the low temperature. The position of the tropical belts 
of high pressure is a resultant of the high temperature of the equatorial 
regions on one side and the polar whirls on the other. 



Fig. 283. 



DR. PHYS. GEOG. — 19 



CHAPTER XXVII 
STORMS 

Cyclones. — The regularity of the general system of 
prevailing winds is subject to local and temporary dis- 
turbances called storms. A storm is usually characterized 
by an increase of wind velocity, accompanied by precipita- 
tion. A large majority of storms are cyclonic whirls in 
which the air moves spirally toward a center of low pres- 
sure. The isobars are seldom circular, but extend in more 
or less elliptical curves around the low center. The 
general course of the winds is across them, but at a smaller 
angle as the center is approached, as shown in Fig. 279. 
The motion becomes more rapid and more nearly circular 
as the air rises around a central calm. The cyclonic or 
vortex movement may be regarded as. the normal air move- 
ment on a rotating earth. Each of the great polar whirls 
in the upper air covers half the earth. Within these are 
smaller cyclones of the second, third, and even fourth 
order, down to little dust whirlwinds a few feet in diame- 
ter. The temperate or mid-latitude regions of the northern 
hemisphere are much frequented by cyclonic storms which 
are of great extent, but not violent. They often attain a 
diameter of more than a. thousand miles and bring their 
characteristic weather conditions to a correspondingly 
large area. While the movement of the air at every 
point within their circumference forms a part of a cyclonic 
system, the center of rotation moves forward in a general 
easterly direction at the average rate of about thirty miles 
an hour. Thus the whirl travels through the atmosphere 



STORMS 



313 




Fig. 284. — Upward movement of air 
at center of a cyclone. 



as an eddy moves through still water, constantly taking in 

new air in front and dropping out air behind. The air 

rises as it approaches the 

center, and at the height of a 

mile or more spreads out in a 

reverse direction toward the 

circumference. 

The maps, Figs. 286-288, show 
the progress of a cyclonic storm 
across the United States and the weather conditions which accompany 
it. As" in this example, the isobars encircling the center of low pressure 
are usually more or less elongated in a north-south direction. The 
result is that in the front or eastern half of the storm the prevailing 
winds are from the southeast and south, and in the rear or western half 
from the northwest and north. Over a small area on the north side 
east winds occur, and on the south side west winds. The southerly 
winds coming from the Gulf of Mexico and Atlantic are warm and damp, 

and- as they advance northward 
are cooled by radiation and con- 
duction. Consequently they bring 
cloudy weather with rain or snow, 
which on account of the rising 
and mixture of air around the 
center often extends over a large 
area on all sides. On the west 
side the northerly winds coming 
from the interior of the continent 
are cool and dry, and as they 
advance southward are warmed. 
Consequently they evaporate the 
clouds and bring clear weather. 
As the storm advances, these 
two strongly contrasted types of 
weather prevail in succession at 
lg ' 2 5 ' every point in its path. Fig. 285 

shows the curves of pressure in a cyclone ; notice that the changes of 
pressure are greater and more rapid along BA than along ED. See 
also Fig. 301. 




STORMS 315 

Anticyclones. — The areas of relatively high pressure 
between the cyclones sometimes take the form of irregular 
"ridges," but more often they appear as definite centers of 
high pressure, or anticyclones, as in Figs. 286-288. The 
conditions in an anticyclone are the exact reverse of those 
in a cyclone, as shown on p. 306. At the center the air is 
descending, and when it reaches the surface of land or water 
it spreads down the pressure slope in all directions. The 
rotation of the earth gives it a spiral motion, clockwise in 
the northern hemisphere. The anticyclonic centers move 
eastward along paths similar to those of cyclones. On 
the eastern side the winds are chiefly from the north and 
northwest, and bring cold, clear weather. On the western 
side the winds are chiefly from the south and southeast, 
and bring higher temperature to the regions over which 
they blow ; but owing to the fact that these currents are 
supplied with dry air which descends from above at the 
center and is warmed by compression, they do not usually 
bring cloudy or rainy weather, as do the winds in front of. 
a cyclone. 

Warm and Cold Waves. — The southerly winds in front of a cyclone 
carry the isotherms northward, as shown in the eastern part of the 
map, Fig. 287. As the cyclone advances eastward, it carries a wave 
of rising temperature in front of it. This effect, however, is not usu- 
ally so pronounced as the cold wave which precedes an anticyclone. 
The northerly winds in front of an anticyclone cause the isotherms to 
curve away from the center of high pressure, as shown in Figs. 289- 
291. When a cyclone passes across the southeastern part of the 
United States, followed by an anticyclone in the northwest, a wave of 
falling temperature spreads over the greater part of the country. In 
winter freezing temperatures may be carried nearly to the tropic and 
zero weather to the Gulf states. In the northwest the air is sometimes 
filled with extremely fine ice crystals driven by a high wind. Such a 
storm is locally known as a blizzard. In the north and east, the storm 
usually brings a heavy fall of snow. 



.5? o 






316 




STORxMS 317 

The Procession of Cyclones and Anticyclones. — Through 
the greater part of the year, but especially in the winter 
months, the eastern part of North America is traversed 
by a more or less 
irregular but continu- 
ous procession of 
cyclones and anticy- 
clones which succeed 
one another at inter- 
vals of a few days. g ' 292 
The result is a rapid succession of weather changes, 
which are often sudden and decided in character. If 
the cyclones and anticyclones all pursued the same path 
at regular intervals, the result would be as shown in Fig. 
292. An approach to this condition appears upon the 
maps, Figs. 286-288, but it usually happens that the paths 
of the different centers are not uniform and the spacing 
between them is unequal. Thus the order of their occur- 
rence is irregular. Cyclones and anticyclones also vary 
greatly in development. Some are small and feeble, some 
large and strong, and the degree of control which they 
exercise over the weather conditions varies accordingly. 

Figures 286-288 show the progressive development of a temperate 
cyclone. On 286 the low center in Montana, with a. pressure of 29.7 
inches, is surrounded by only three isobars, which are far apart. The 
pressure slope is gentle, and the winds are light and irregular. On 287, 
the center has moved to Illinois, with a pressure of 29.6 inches, and is 
surrounded by five isobars. The slope is steeper, the winds are stronger 
and show very little variation from a regular spiral whirl. On 288, 
only the rear half of the cyclone is shown, but the pressure at the 
center, now off the New England coast, has fallen to 28.8 inches and 
it is surrounded by fourteen isobars, which are closely crowded. The 
slope is very steep, and the velocity of the wind is high, amounting 
near the center to a gale dangerous to shipping. The crossed line 
shows the path pursued by this cyclone across the United States and 



3 i8 



THE ATMOSPHERE 



the position of its center from day to day. The maps, Figs. 289-291, 
show a cyclone which developed very rapidly, having on the second day 
nineteen isobars, and a difference of pressure between center and cir- 
cumference of 1.80 inches. Two low centers may combine into one, or 
a single one may break up into two. Cyclones usually pass off into 
the Atlantic Ocean, where they gradually die out, or they may con- 
tinue across Europe as far as central Asia. 

Storm Paths. — On Fig. 293 the paths of many individual 
cyclones are shown. The heavy line marks the path of 

the greatest number. Fig. 
304 shows favorite paths 
across the United States. 

Weather Maps. — The daily 
weather maps issued by the 
United States Weather Bureau 
should be consulted for exam- 
ples of cyclones and anticy- 
clones. The maps for January, 
February, and March furnish 
the best and most numerous 
specimens. The local weather 
conditions as observed by the 
student should be compared 
each day with those shown by 
the weather map. Observation 
and map study carried on to- 
gether for two or three months 
will make clear the laws which 
govern the apparently capricious 
changes of the weather, and will enable the student to predict those 
changes with a fair degree of accuracy. Weather maps may be obtained 
by addressing the local officer in charge of the nearest observing station 
(see Appendix, p. 411). 

Tropical Cyclones. — In the region between the tropics, 
cyclones occur which are much smaller than those of 
temperate regions, but are of proportionately greater vio- 
lence. They are developed over the western parts of the 




equator 



Fig 293 



STORMS 



319 



TO 80 90 100 HO 120 130 140 




Fig. 294. — Paths of typhoons. 



oceans, those in the north Atlantic being called hurricanes, 
and those of the Pacific and Indian, typhoons. They orig- 
inate in the belt of 
equatorial calms when 
farthest from the equa- 
tor. From a small be- 
ginning they increase 
to a diameter of 300 to 
500 miles. At the same 
time the velocity of the 
wind increases to a de- 
gree which becomes 
destructive to shipping 
upon the seas and to 
buildings, forests, and 
crops on land. After a career which lasts many days or 
even weeks, they gradually die out. Their paths are more 
uniform and regular than those of temperate cyclones. 
They move westward and poleward at right angles to the 

trade winds, until they 
reach latitude 25 to 30 , 
where they turn rather 
abruptly into the path of 
the antitrades and move 
poleward and eastward. 

West India Hurricanes. — In 
the months of August, Septem- 
ber, and October the West 
India Islands are subject to 
cyclones which arrive from the 
east and southeast, and depart 
toward the northeast along or 
near the coast of the United States. The winds acquire a spiral move- 
ment which becomes nearly circular around a central region of calm 




Fig 295. —Paths of hurricanes. 



320 THE ATMOSPHERE 

which is from ten to twenty miles in diameter. The circumference of 
the storm is marked by a slight rise of the barometer and the appear- 
ance of fine cirrus clouds which form in the air, blowing out from the 
top of the approaching whirl. The barometer begins to fall, the wind 
freshens, and the cloud mass becomes more dense. As the center 
comes nearer, the wind rises to a gale, and the clouds gather into a 
black mass of nimbus from which heavy rain falls. Within about fifty 
miles of the center, the barometer sinks rapidly, the wind attains full 
hurricane strength, the clouds are so dense as to change daylight into 
the darkness of midnight, and the rain pours down in torrents, accom- 
panied by frequent flashes of lightning. At the center of the whirl 
these conditions change very abruptly. The wind falls to a calm, the 
rain ceases, and the clouds break away, showing a clear sky. The 
barometer now reaches its lowest point, which may be less than 27 
inches. This calm, clear, central space is called the eye of the storm, 
and it may occupy an hour or two in passing. Then the hurricane 
begins again with sudden and extreme violence, but the winds are re- 
versed in direction. All the phenomena observed in the first half of 
the storm are repeated in reverse order but in somewhat more rapid 
succession. The barometer rises, the violence of the wind gradually 
abates, the rainfall becomes more gentle, the dark nimbus clouds lighten 
and at last disappear, the lofty cirrus clouds recede, and the storm has 
passed away. 

Destructiveness. — Tropical cyclones are much dreaded by ship cap- 
tains. When the warning signs of their approach are observed, the 
ship is put upon a course which usually takes it out of the path of 
the central portion. If caught in the most violent part of the storm, it 
is liable to be wrecked by the force of the winds and waves, and can 
hardly escape without serious injury. In passing over the land the 
hurricane causes great destruction to life and property. Hardly any- 
thing of value is left in its path. The smaller islands are sometimes' 
literally swept clean of trees, crops, buildings, and almost of popula- 
tion. Perhaps the most complete ruin is accomplished along the coast, 
which suffers from the combined action of wind and wave ; for the low 
atmospheric pressure at the storm center and the inblowing winds 
cooperate to produce a heaping up of the water to a height of many 
feet above the usual sea level. 

Causes of Tropical Cyclones. — The evidence seems to 
point clearly to the conclusion that a tropical cyclone is 



STORMS 321 

a part of a system of convection currents set up and main- 
tained by a difference of temperature and subject to the 
influence of the earth's rotation. Under the direct rays 
of the tropical sun the lower air becomes excessively 
heated and in contact with the sea excessively humid. 
It is thus made less dense than the air which overlies 
it — a condition which is as unstable as would be a 
layer of oil under a layer of water. Sooner or later the 
lighter air below breaks through the heavier air above and 
drains away upward like a draught in a chimney. The sur- 
rounding air crowds in from all sides toward the bottom 
of the updraught and soon acquires the usual spiral motion. 

As the air currents approach the center the rapidity of rotation be- 
comes so great that centrifugal force overcomes gravitation and prevents 
the incoming winds from reaching the center, which is left as a calm 
and comparatively emptied of air, like the core in the center of a water 
eddy. The air escapes by a spiral movement upward, and since there 
is no more efficient cause of cooling and condensation than the expan- 
sion of rising currents (see p. 282) the great mass of nimbus cloud and 
the downpour of rain necessarily follow. In the eye of the storm the 
air is not rising and there is probably even a slight downward draught, 
which tends to produce a clear sky. 

Duration and Force. — The amount of energy required to maintain 
the high velocity of a hurricane in a mass of air 300 miles in diameter 
is enormous and can not be derived from the original heat energy which 
started the updraught. One typhoon has been known to continue for 
thirty-five days and to travel the whole length of the heavy line on 
Fig. 293 from the Philippine Islands to central Europe, a distance of 
more than 14,000 miles. A very large supply of energy is derived from 
the liberation of latent heat which accompanies the rapid condensation 
of water vapor in the rising column. Thus the cyclone maintains at 
its own center a virtual furnace which keeps up the temperature as long 
as water vapor is supplied for condensation. When it passes over the 
land and is fed with dry air, it rapidly loses force and is finally over- 
come by friction. 

Course. — The westward and poleward course of a cyclone within the 
tropics is probably a resultant of two forces. Its lower portion is in 



322 THE ATMOSPHERE 

the current of the trade winds, which tend to carry it westward, while 
its upper portion rises into the current of the antitrades, which tend to 
carry it poleward. The result is movement in a direction between the 
two. Beyond the tropics it follows the northeastward or southeast- 
ward drift of the antitrades. 

Origin of Temperate Cyclones. — The conditions under 
which temperate cyclones originate are so different from 
those which prevail at the birthplace of tropical cyclones 
that it seems impossible to attribute them to similar causes. 
Temperate cyclones are more frequent and violent in win- 
ter than in summer. Many of them are developed over 
land where the air is dry and cold, and the conditions are 
unfavorable to convection. The theory that temperate 
cyclones are eddies set up around the margin of the polar 
whirl is a plausible one. The map on p. 309 shows that 
the north polar whirl in winter is divided into two whirls 
around the centers of low pressure in the north Atlantic 
and north Pacific oceans. The margin or circumference 
of the whirl is along the axis of the belt of high pressure 
which surrounds these areas of low pressure, and Fig. 293 
shows that the most frequent path of cyclones nearly 
coincides with this axis. The Atlantic whirl seems to be 
more prolific of cyclones than the Pacific. The oblique 
flow of the upper and lower winds into the ever narrowing 
space around the pole, the return of the air at intermediate 
levels, and the friction of continents may well give rise to 
local crowding and disturbance, and the temperate cyclones 
may be eddies driven by the general winds, like the eddies 
produced in a river where its banks and bottom are 
irregular. 

Form of Temperate and Tropical Cyclones. — These whirling masses 
of air are not tall and slender columns as we are apt to imagine, but rel- 
atively thin, flattened disks, not more than five miles in thickness and 
from 300 to 1500 miles in diameter. A circular disk one inch in diam- 



STORMS 



323 



eter cut from a leaf of this book would not be too thin to represent 
their average proportions. 

Tornadoes. — A tornado is a small and violent cyclone 
which appears as a funnel-shaped cloud with the small 
end down. Its formation or approach is preceded by the 
rapid movement of cloud masses toward some central point. 
The clouds may look 
as if lighted up by 
a great fire or like 
dense volumes of 
smoke, or may have 
a peculiar greenish 
hue. The funnel- 
shaped cloud de- 
scends as a pendant 
from the larger 
cloud mass, like an 
elephant's trunk, 
and dangles above 
or upon the ground, 
writhing and twist- 
ing about, touching 
here and there, and 
often skipping over 
a portion of its regu- 
lar path. The tor- 
nado travels toward the northeast, in the northern hemi- 
sphere, at the rate of about forty miles an hour, and 
seldom continues more than two hours. Along a path 
which varies in width from a few rods to half a mile, it is 
extremely destructive. The velocity of the wind in the 
whirl often reaches 200 miles per hour and occasionally 
twice as much. Nothing except the solid earth itself' can 




Fig. 296. — How a tornado looks. 
(Woods County, Okla., May 18, 1898.) 



324 



THE ATMOSPHERE 



withstand its force. It creates a deafening roar like the 
rumble of a railroad train over a bridge, greatly intensified. 
In front there is a gentle southerly breeze or a dead calm 
with oppressive heat. The tornado passes in a minute or 
two and is followed by a sudden fall of temperature. 

Through a forest a tornado cuts a swath like that of a mower through 
a meadow, the trees being twisted off or uprooted. From plowed fields 
it removes the loose soil, and sucks up the water from small ponds, leav- 
ing them dry. Even large boulders and masses of iron are taken up 
and transported hundreds of feet. Buildings of all kinds which stand 
in its way are demolished, and their fragments scattered over the sur- 
rounding country. Animals and human beings are lifted and whirled 
about and sometimes transported a half mile or more. They are often 
killed by flying debris and sometimes seem to be literally torn in pieces. 
Heavy structures are removed from their foundations and locomotive 

engines lifted from the rails. The 
smaller work of a tornado is equally 
impressive, such as the stripping of 
feathers from fowls and the cloth- 
ing from persons. Wire hairpins 
have been driven through fence 
boards and straws driven into oak 
wood. Almost any story of a tor- 
nado's energy may be true, because 
the truth is beyond the power of human imagination to invent. A 
part of a house may be reduced to fragments while the rest is left un- 
disturbed. People have been carried long distances and deposited 
unhurt. Heavy objects are removed and light and fragile ones left in 
place. Fragments of furniture from the same room or from the same 
piece are often widely scattered in opposite directions. These and 
other mysterious freaks are probably due to irregular and confused cur- 
rents in the general whirl. The walls of buildings are often thrown 
outward as if by an explosion from within. 

Tornadoes always occur some hundreds of miles to the southeast of 
the center of a temperate cyclone, where a current of warm, moist air is 
underrunning a layer of colder air. They occur chiefly in the summer 
months and in the afternoon of hot days. These conditions are very 
favorable for the starting and maintenance of strong convection currents. 




Fig. 297. — Pressure during the passage 
of a tornado. 



STORMS 



325 



Tornadoes seldom occur singly, but in groups of three or more, which 
follow parallel paths. As many as forty have been reported from one 
locality in one day. The av- 
erage number in the United 
States is about 150 per year. 
They are most frequent in Kan- 
sas, Iowa, Missouri, Illinois, 
and Georgia, and are almost 
unknown north of the forty- 
fifth parallel and west of the 
one hundredth meridian. 

Spouts. — A tornado at sea 
takes the form of a whirling 
column which extends from the 
clouds to the water surface and 
is called a waterspout. It is 
formed by the usual tapering 
funnel, which descends from 
the clouds and is met by rising 
water below. The greater part 



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Location of tornadoes in a cyclone. 
(Shown thus: x x x.) 



of the column, however, is composed of cloud and rain. When it passes 
over a ship, as occasionally happens, there is a deluge of fresh water. 

In the desert columns of whirling sand are of frequent occurrence 
and are maintained for several hours. The small whirlwinds common 
on dry, warm days are worthy of careful observation, since they present 
many of the essential features of cyclones on a small scale. 

Thunderstorms. — The rapid condensation of water vapor 
is often accompanied by the generation of electricity, and 
when this occurs to such a degree as to produce frequent 
discharges of lightning from cloud to cloud or between the 
clouds and the earth, the disturbance is called a thunder- 
storm. A local ascending current of warm, moist air de- 
velops at its summit a cumulus cloud with a flat base. This 
increases in height and area for several hours, until rain 
begins to fall. The air is cooled and pressed downward 
until the current in the central portion is reversed. The 
column of descending air spreads out at the bottom and 



326 THE ATMOSPHERE 

becomes surrounded by a current of ascending air which 
continues to supply moisture to the cloud above. At the 
center the pressure is high and the temperature low ; at 
the circumference these conditions are reversed, and be- 
tween the two there is a zone of strong contrasts and steep 
gradients marked by squalls of violent wind and rain. 



lllll nil —Hill fTlm 




Fig. 299. —Clouds and winds in a thunderstorm which is moving toward the right. 

Progressive Thunderstorms. — A thunderstorm is very apt to take on 
a progressive movement in the direction of the general air current (east- 
ward in the United States), and to' broaden out so as to present a convex 
front, which increases in length. The cloud mass may attain a length 
of 100 miles and a breadth of 30 miles, and reach to a height of 5 
miles. Its front edge of cirro-stratus, with rolling festoons of cloud 
below, extends from 10 to 50 miles in advance of the rain. The rate of 
movement is from 20 to 50 miles an hour, and it may continue from 2 
to 12 hours. As the storm approaches, the sky is gradually overcast, 
the air is hot, breathless, and oppressive, the barometer falls, and the 
distant thunder is heard. In front and below there is a strong out- 
rush of cool air which lasts but a few minutes and is followed by the 
dash of rain. The temperature falls rapidly, sometimes as much as 
twenty degrees in a half hour, the barometer jumps suddenly upward, 
and the darkness of the downpour is broken by vivid flashes of light- 
ning. The rainfall seldom lasts more than an hour unless a second 
storm follows close upon the first. 

Cloudbursts. — In tornadoes or thunderstorms strong ascending cur- 
rents may carry up and sustain the rain or hail until an excessive 
quantity has accumulated aloft, which sooner or later falls in an almost 
solid mass of water. Such events are popularly known as cloudbursts. 



CHAPTER XXVIII 

RAINFALL 

Causes of Rainfall. — The general causes and conditions 
which promote condensation of water vapor and the fall of 
rain and snow have been discussed in Chapter XXIII. The 
distribution of rainfall over the earth remains to be con- 
sidered. On account of the absence of permanent observ- 
ing stations at sea, the rainfall has never been accurately 
measured there. The facts as observed upon land are 
shown on maps, pp. 328, 330, 331. The conditions neces- 
sary for considerable rainfall anywhere are (1) a large 
body of water from which sufficient evaporation may 
occur, (2) air currents to transport the vapor over the 
land, and (3) some agency for cooling and condensing the 
vapor. The first requisite is supplied almost solely by 
the sea, the second by prevailing winds, and the third by 
winds and by elevations of the land. On account of the 
intimate relations between the winds and the supply of 
moisture, each wind belt is characterized by a peculiar 
type of rainfall, while the varied relief of the land breaks 
up the belts into more or less distinct and contrasted por- 
tions. Hence the patchwork appearance of the maps. 

Equatorial Rains. — In the region between the tropics 
the rainfall is generally large and is the result of two proc- 
esses : (1) the rising of the air in the equatorial calm belt, 
and (2) the flow of the trade winds from the ocean over 
the land. 

In the belt of equatorial calms, heavy rains are of almost daily 
occurrence, and are produced by the mechanical cooling of rising air. 

327 




ft «• 



RAINFALL 329 

The mornings are usually clear, but cumulus clouds soon begin to form, 
which continue to grow until afternoon, when thunderstorms occur, often 
succeeding one another into the night. These conditions accompany 
the equatorial belt of low pressure in its migrations and therefore pass 
over the regions within its range twice a year. Near the northern and 
southern limits reached by the belt the two rainy seasons merge into 
one and occur in summer when the sun is nearly vertical. At the 
middle of the belt, rainy seasons occur in the spring and fall. Those 
months during which the trade winds blow without interruption are dry 
except where winds from the sea strike the side of a plateau or moun- 
tain range and are compelled to ascend. 

In January, on account of the low pressure over the southern land 
masses, the rains extend beyond the southern tropic in South America 
and Africa and reach the northern coast of Australia. In July they 
cross Central America, the West Indies, and central Africa, and are 
carried by the monsoons to the coast lands of Asia from India to 
Japan. Over the regions between these limits, with few exceptions, 
the rainfall ranges from 40 to over 80 inches per year. In South 
America there is an excess on the northeast and southeast coasts due 
to the highlands, which extend across the course of the trade winds 
and act as very efficient condensers. The wide extent of heavy rainfall 
over the lowlands of the Amazon basin is probafbly due to the dimin- 
ished speed of the trade winds by friction in passing over the land. 
The currents from the ocean supply more air than can pass in a layer 
of uniform thickness, and the air is compelled to rise, as the surface of 
a stream of water is raised in passing over an obstruction in its bed. 
On the west coast of South America there is a deficiency of rainfall, be- 
cause the lofty chains of the Andes permit little moisture to pass over 
them. In Africa there is a deficiency in the eastern peninsula where 
the monsoons blow from the land. In southeastern Asia there is a 
large excess of rainfall in summer, but on account of the varied relief it 
is unequally distributed. The southwest coasts of India and Indo- 
China and the Ganges and Brahmaputra basins receive excessive rain 
(80-400 inches), m<5st of which falls in'The summer; while over the 
Dgkka'n plate|u*there is a deficiency. The Khasia hills, north of the 
Tiead qi^he Day of Bengal, receive the heaviest rainfall in the world, 
^a^praging 493 inches per ^ear and amounting in some years to 600 
inches. Over 438 incnes falls in five summer months, and more than 
40 inches has^ieen recorded in a single day. 
w 




33° 



332 THE ATMOSPHERE 

Tropical Rains. — In the tropical belt of high pressure 
the air is warmed by compression as it s descends, and 
hence brings little rain. As it shifts north and south, it is 
followed by the trade winds on the equatorial side and by 
the temperate cyclones on the polar side. 

The northern tropical belt is much less regular and well-defined than 
the southern. In summer it nearly ceases to exist over the land, and 
its place is taken by the trades. Where these blow from the ocean, 
against highlands, as in Central America, they bring a wet season ; but 
where they blow from the land, as in the Mediterranean region and 
southwestern Asia, they bring a dry season. By a combination of 
these conditions, southwestern United States, north Africa, Arabia, 
Persia, and the region of the Caspian and Aral seas are perennially 
dry. Of these regions, the Sahara and Arabian deserts are the largest 
and driest in the world. 

The southern tropical belt in winter crosses southern South America, 
south Africa, and central Australia, causing a deficiency of rainfall at 
that season. A narrow strip on the coast west of the Andes in northern 
Chile forms the desert of Atacama, and a larger area in southwest Africa 
the Kalahari desert, while central Australia contains the largest desert 
in the southern hemisphere. In summer the tropical belt lies to the 
south of all the land except South America. 

Rains of Middle Latitudes. — Beyond the tropical belts of 
high pressure rains are brought either by the westerly anti- 
trades or by temperate cyclones ; consequently the rainfall 
varies more along east-west than along north-south lines. 

In winter the west coast of North America has abundant rainfall as 
far south as 35 , but on account of the mountains near the coast it 
extends but a few hundred miles inland. The coast of southern Alaska, 
being reached by perennial southwest winds, has rain at all seasons, but 
south of 40°, on account of the cessation of southwest winds in summer, 
rain is scant or wanting in that season. Central North America, from 
Mexico to the Arctic Ocean, is dry, partly from being too far inland, 
and partly on account of the mountains along the Pacific coast. The 
Rocky, Wasatch, and other mountains and plateaus which rise above 
8000 feet receive a moderate rainfall. Eastern North America from 
the Gulf of Mexico to Hudson Bay enjoys a rainfall above 30 inches, 



RAINFALL 333 

increasing to 60 inches on the Gulf coast. This is due chiefly to the 
cyclonic storms which bring moisture from the south and southeast, 
and to the absence of elevations sufficient to shut it out. The Gulf of 
Mexico appears to furnish the larger quantity, but the southerly winds 
are fed in summer by the northeast trades from the Atlantic and sweep 
far inland toward the continental center of low pressure. The rainfall 
is well distributed throughout the year, but is generally heavier in spring 
and summer than in autumn and winter. The increased frequency of 
cyclones in winter nearly compensates for the lower absolute humidity 
of the air. The largest mean annual rainfall in North America is at 
Sitka, Alaska, 112 inches, and at Neah Bay, Washington, 105 inches; 
while the Mohave desert, California, has the smallest recorded rainfall 
in the world, 1.85 inches. 

The rainfall of Europe and northern Asia is chiefly supplied by the 
westerly winds from the Atlantic. It therefore decreases from west to 
east and is greatest in summer and autumn, when the center of low 
pressure over Asia causes the winds to penetrate farther inland. North- 
western Europe has a rainfall well distributed throughout the year, with 
a slight excess in autumn and winter, due to the center of low pressure 
in the north Atlantic and the greater frequency of cyclonic storms. 
The rainfall gradient is steep in the British Isles and Norway, some 
places on the west coast having seven times as much rain as places one 
or two hundred miles to the east. 

The high table-lands of central Asia are chiefly desert on account of 
the surrounding rim of mountains. 

In southern South America the west winds bring ample rainfall to 
a narrow strip on the Pacific slope of the Andes, but it diminishes 
rapidly eastward, and the plains of Patagonia are arid. 

During the southern summer (January) the southeast trades bring to 
southeast Africa and to Australia moisture, which is condensed by the 
highlands near the coast. 

General Laws of Distribution. — In spite of the great 
irregularity of distribution of rainfall, four general laws 
may be observed. 

(1) Rainfall decreases from the equator toward the 
poles. 

(2) Rainfall is generally less in the interior of a con- 
tinent than on the coasts. 

DR. PHYS. GEOG. — 20 



334 THE ATMOSPHERE 

(3) In trade-wind regions (between 35 south and 40 
north) rainfall is greater on east coasts than on west. In 
antitrade-wind regions the reverse is true. 

(4) Rainfall increases with altitude up to a certain ele- 
vation, which varies in different regions, and then dimin- 
ishes. On mountain ranges the rainfall is greater on 
windward slopes. The position of high ranges is well 
marked on the rainfall map. 

Although no known regions are absolutely rainless, 
about 20 per cent of the land surface has less than 
ten inches of rain per annum, and is consequently in a 
desert condition, while nearly 50 per cent has less than 
twenty inches, and is generally unfitted for agriculture 
without irrigation. 



CHAPTER XXIX 
WEATHER AND CLIMATE 

Weather. — The conditions of the atmosphere at any- 
given time constitute the weather. It includes pressure, 
temperature, humidity, state of the sky, precipitation, and 
direction and force of the wind. The most prominent 
characteristic of weather is its changeableness, the exact 
continuance of any given combination being hardly more 
than momentary. Weather changes are chiefly of three 
classes : ( i ) daily changes due to the rotation of the earth 
upon its axis, (2) yearly seasonal changes due to the 
revolution of the earth around the sun, and (3) irregular 
changes due to the passage of storms. The daily changes 
bring relatively warm days and cool nights, the yearly 
changes bring more or less decided variations of average 
daily and monthly temperature and rainfall, and the irregu- 
lar changes bring alternations of fair and stormy weather. 
These various kinds of changes are of very different 
prominence and value in different parts of the earth. 

Climate. — The average succession and distribution of 
weather conditions at any given place constitute the cli- 
mate of that locality. The climate of a place can be 
determined only by taking into account a period of at 
least ten years. The factors of climate and the different 
methods of grouping them are very numerous. The most 
important factors are (1) the mean annual temperature, 
(2) the annual range of temperature, (3) the mean annual 
rainfall, and (4) the distribution of rainfall through the 
year. 

335 



336 THE ATMOSPHERE 

The mean annual temperatures of New York and of London are 
nearly the same, but the range at New York is 40 , while at London 
it is only 20 . The rainfall of Portland, Ore., is forty-seven inches 
and of Portland, Maine, forty-two inches ; but in Oregon two thirds of 
it falls in five winter months, while in Maine it is almost equally dis- 
tributed through all the months. 

Weather and climate are so closely related that they are 
studied to the best advantage together. The distribution 
of temperature and rainfall has been discussed in previous 
chapters, but it remains to consider their combinations 
with each other and with subordinate factors which deter- 
mine the various climates of the globe. 

Zones of temperature are well defined by isotherms as given on 
p. 295. Zones of rainfall bear close relation to the belts of pres- 
sure and prevailing winds, but their boundaries are vague and shifting, 
and each zone is far from presenting uniform conditions throughout. 
To map out zones which represent the distribution of the still more 
complex phenomena of climate is a difficult matter. The continents 
and oceans extending north and south cut across all zones and break 
them up into strongly contrasted portions. In discussing climatic zones 
sharp distinctions, definite boundaries, and uniform conditions must 
not be looked for. 

The Trade or Constant Zone. — Figure 270 shows that the 
northern and southern limits of the swing of the isotherm 
of 70 , or the limits of hot summers, correspond quite 
closely with the tropical belts of high pressure and the 
limits of the trade winds. If the January isotherm of 30 
is substituted for the July isotherm of 70 across North 
America and from central Europe to Japan, these boun- 
daries will be near the parallels of 30 south and 40 
north latitude, and mark out a zone of tolerably uniform 
climatic conditions. The zone bounded by these parallels 
is characterized as a whole by high and relatively uniform 
insolation amounting everywhere to more than 80 per cent 
of that at the equator, by prevailingly high temperatures 



WEATHER AND CLIMATE 337 

generally above 6o°, by constant winds, by excessive rain- 
fall, by an absence of strong seasonal contrasts, and by 
uniformity of weather conditions from day to day. 

While the trade winds blow the daily changes are more prominent 
than any other, and the daily range of temperature may be greater than 
the yearly. The curves of temperature and pressure show their daily 
maximum and minimum with scarcely any variation from week to week. 
The sky is clear or partly cloudy in the daytime, and rain falls, if at all, 
in the latter part of the day. Cyclones rarely occur, but are extremely 
violent. The lowlands of the trade-wind belts are largely desert or 
semi-arid except on east coasts, and include the Sahara, Arabian, 
Australian, and South African deserts. The wind in these deserts is 
excessively dry and laden with dust. On account of the absence of 
cloud and moisture radiation is unchecked. The temperature at mid- 
day may rise to uo° or 120°, but at sunset the wind subsides and the 
air soon cools to temperatures not far above freezing. 

The trade-wind belts are separated and modified by the 
belt of equatorial calms. As it swings back and forth it 
carries with it a calm, hot, moist air with abundant cloud 
and daily rainstorms. These conditions continue at any 
given place for two or three months, constituting the rainy 
season. Their most important results appear in the heavy 
rainfall and dense forests of equatorial Africa, equatorial 
South America, and the East Indian archipelago. 

The Asiatic monsoon region is a peculiar and divergent 
offset from the trade-wind zone, subject to both extremes 
of wet and dry. In one season of the year the climate 
resembles that of the Sahara, and in the other that of the 
Amazon basin. In winter the weather is cool and dry, 
the northerly trade winds being occasionally interrupted 
by a mild cyclonic storm. The spring is dry and hot, but 
by June the southerly monsoon has become established and 
brings a copious rainfall which continues until October. 

Transition Areas. — Certain limited areas along the borders of the 
trade-wind zone are included in that zone in summer and in the anti- 



338 THE ATMOSPHERE 

trade zone in winter. Among these are southern California, the coast 
lands of the Mediterranean, and southern Australia and Africa. They 
have a summer temperature between 65 and 8o°, and a winter tempera- 
ture between 50 and 6o°, the annual range being less than 30 . They 
are too far from the equator to receive the equatorial rains and too near 
it to be visited by the cyclonic storms and cold waves of middle latitudes. 
In summer they have uninterrupted fair weather and in winter a light 
rainfall. Their climate is among the most enjoyable in the world, and 
in the northern hemisphere they have long been famous as health resorts. 

The Southern Antitrade Zone. — Between the tropical 
belt of high pressure and the Antarctic circle the face of 
the earth is occupied by the sea, interrupted only by New 
Zealand and the narrow extremity of South America. This 
zone presents, therefore, in a high degree, the characteristic 
oceanic climate of middle latitudes. The westerly winds 
are almost as steady as the trades, but are much stronger. 
The ocean currents wheel perpetually with the winds 
around the polar center, and their circuit is joined in only 
two or three places by streams of air or water from the 
north. Cloud, fog, and storm are the rule at all seasons. 
The mean annual temperature is low and the daily and 
yearly ranges are small. The chief difference between the 
seasons is the greater frequency and strength of cyclonic 
storms in winter. There is neither summer nor winter, in 
the northern sense of the words, but an alternation, at 
intervals of two or three days, of more and less stormy 
weather which may bring rain or snow in any month of 
the year. The southern zone is truly temperate in that 
its climate is singularly free from extremes of any kind and 
uniform in its distribution over the whole zone. It is also 
truly variable in that weather changes are frequent although 
not of great magnitude. Here great variations of insola- 
tion are almost overcome by the equalizing effect of a large 
body of water. 



WEATHER AND CLIMATE 



339 



The Northern Antitrade Zone. — The middle latitudes of 
the northern hemisphere are occupied by the largest land 
masses on the globe, and consequently possess a climate 
in strong contrast with that of the corresponding south- 
ern zone. The northern zone is truly intemperate, in that 
its climate is characterized by extremes of all the factors 
and by a want of uniformity in their distribution. Extremes 



UNITED STATES 43'/2°LAT. EURASIA 52° LAT. 









































































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West Coast Interior — fast Coast 



Fig. 300. 

prevail through so large a part of the year that annual 
averages lose their value. The seasons of maximum and 
minimum temperature are separated by well-defined tran- 
sition periods. The occurrence of four distinct seasons 
of nearly equal length is peculiar to this zone. Daily 
changes are most prominent in summer, and irregular 
cyclonic changes in winter. On account of the different 
heat relations of land and water (see p. 296), the isotherms 
over the land bend far southward in winter (see Fig. 268) 
and far northward in summer (see Pig. 269). Over the 



340 THE ATMOSPHERE 

oceans reverse bends are equally decided. This irregular- 
ity is intensified by a cold current on the west side of each 
ocean and a warm current^ on the east side. West coasts 
of the land are swept by winds from the ocean and have 
an oceanic climate. East coasts are swept by winds from 
the continents and have a climate similar to that of the 
continental interiors. Under the combined influence of 
these conditions the isotherms near either coast extend 
almost parallel with it, especially in winter, and the tern- j 
perature varies more rapidly along east-west than along 
north-south lines. Another result is the great range of 
temperature in the interior of the land masses (see map, 
p. 299). The differences of climate between the west coasts 
and the interiors rival in magnitude those between the 
equator and the poles. 

West Coast Climate. — On the western coasts of North America and 
Europe, in the antitrade zone, the climate is almost as equable as that 
of the trade wind belt and less stormy than that of the south temperate 
zone. The air is damp and more than half the days are cloudy. In 
winter there is an almost constant fog and drizzle of rain, with snow on 3 
the higher elevations. The winds are more northwesterly in summer 
and more southwesterly in winter. There is an excess of rainfall in 
winter. The winds from the ocean and the prevailing cloudiness com- 
bine to prevent great or sudden changes of temperature. The lines of 
equal annual range run parallel with the coast, the range slowly increas- 
ing northward from 20° to 40 in America and from 15 to 30 in Europe. 

Eurasia. — On account of the strength and constancy of the south- 
west winds over the north Atlantic and the great mass of warm water 
driven by them in the Gulf Stream, the British Isles and Norway enjoy 
winters of unparalleled mildness for their latitude, and in strong con- 
trast with the east coast of America on one side and the interior of 
Eurasia on the other. To the eastward there is a gradual change from 
these moist and temperate conditions to the dry steppes traversed by 
the Volga and Ural rivers, the arid plains of the Caspian and Aral de- 
pression, the bleak wastes of the Tarim and Gobi deserts, and the sever- 
ity of east Siberian climate, where the lowest winter temperature and 



WEATHER AND CLIMATE 



341 



the greatest annual range on the globe occur in the same province. 
The great ranges of temperature extend with some mitigation to the 
Pacific coast, where the winters are nearly as dry and cold as in the in- 
terior. The summers are cool and rainy, with a southeast monsoon. 

North America. — The region of equable temperature and copious 
winter rains on the west coast of North America is confined to a nar- 
row belt between the sea and the mountains. A day's journey by rail 
would carry the traveler from the mild climate of the coast to the parched 
and burning deserts of Nevada, or the frozen and almost equally dry 
plains of the Mackenzie basin. The interior of North America is sec- 
ond only to central Asia in the intemperate character of its climate. 
The isotherm of 6o° swings from near the tropic in January almost to 
the Arctic circle in July, where the summers are long and warm enough 
to ripen wheat. Summer temperatures above ioo° are common over a 
great part of the region, and the winter temperature falls in some places 
to — 50 , the absolute range reaching 170 on the northern boundary of 
the United States. On the dry and elevated plateaus radiation is rapid 
and the daily range of temperature is very large. In summer violent 
local storms bring the greater part of the rainfall, and in winter the 
whole country from the Arctic Ocean to the Gulf states is liable to be 
flooded for a time with air below zero. North of latitude 50 the sever- 
ity of these conditions extends to the Atlantic coast, but south of that 
parallel they are gradually mitigated toward the southeast. 




•■' ' _" VV'NTEF V.'E .'-HEP CHANGES AT TE 

„,, ' . |a THREE CVCLONE^N^WE^JAI 



Fig. 301. 

The eastern half of the United States lies open to the northwest 
winds from the interior center of cold and high pressure in winter, and 
to the regular southwest antitrades from the heated tropical center of 
Mexico and Arizona in summer. Cyclonic storms are more frequent 
than in any other part of the northern hemisphere, and while they 
impress upon the climate its peculiar variability, they carry moisture 
from the Gulf far inland. From October to May the weather may be 



342 



THE ATMOSPHERE 




Fig. 302— A summer weather map. 

said to be controlled by a succession of cyclones and anticyclones, 
which alternate with each other on an average as often as twice a week, 
and bring each its characteristic type of weather. 

In summer, cyclonic and anticyclonic conditions are much less fre- 
quent and pronounced, the distribution of pressure is often nearly uni- 
form, and the isobars have no definite form or trend. The regular 
southwest antitrade wind blows with considerable strength, bringing a 
steady, moderately high pressure and clear, hot days. The nights are 
calm, clear, cool, and dewy. The barograph and thermograph curves 
show their daily maxima and minima with a regularity equal to that of 
the trade-wind region. These conditions may continue for weeks, but 
are liable to interruption by the passage of moderate cyclones. 



rrr^^^ 




-Of TEMPERATURE AND. PRESSURE AT. T.ERSE HAUTE, IND. 
\ ■ IN SUMMEIR, JUNE 27 TO JULY 4-, IS38i 



Fig. 303. 



WEATHER AND CLIMATE 



343 



Summary of the Climate of the United States. — The 

United States may be divided into three regions which 
exhibit three strongly marked types of climate peculiar to a 
continental area in middle latitudes. 

(i) The Pacific coast enjoys the mild, windward coast 
climate due to the prevailing winds from the ocean. ' The 
winters are warm and the summers cool except in the south- 




Fig- 304. —Cyclone paths and climatic regions of United States. 

ern part. The wet season occurs in the winter and the dry 
season in summer. The rainfall increases rapidly from 
south to north. 

(2) The plateau region is bounded on the west by the 
Pacific mountains and on the east by the rainfall line of 
20 inches and the contour line of 2000 feet, both of 
which lie near the 100th meridian of west longitude. Its 
width is about 1 500 miles, and its average elevation about 
5000 feet. It has a truly continental climate intensified 
by its altitude. The winters are cold and the summers 



344 



THE ATMOSPHERE 




Fig- 3°5- — Average temperature for January in the United States. (After Greely. ) 



hot. In winter the cold is continuous and in summer the 
daily range of temperature is great. This region contains 
the coldest part of the United States, in Montana, and the 
hottest, in Arizona. Great and sudden changes of tern- 




Fig. 306. — Average temperature for July in the United States. (After Greely.) 



WEATHER AND CLIMATE 



345 



7CfV S\\ 
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ll r \/^ } l^- 


^)Tjf — — *~j 
M ( lY 


JBtf 


K%\ 




4<f__ 


130". 

fffr 


■1— *^TL 


\JB[ 3^100° \ 

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K/V \ 
Mao" -^ \ 

, oV ^^L---- 


n I IT 


Cr\ 


JLL 




-\J\ jioo* — j 


lL7n 


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5 


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Ct" 


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2 


\JBO"t\' 80 ° 


— 























Fig. 307. —Absolute annual range of shade temperature in the United States. 
(After Greely.) 

perature are common. The rainfall is deficient, being 
almost everywhere too small for agriculture without irri- 




Fig. 308. — Average annual rainfall in the United States, in inches. 

(U.S. Weather Bureau.) 



346 THE ATMOSPHERE 

gation. It increases from Arizona northward and from 
the Rocky Mountains eastward. 

(3) The lowland region extends eastward from the con- 
tour line of 2000 feet and comprises about one half of the 
United States. Its climate is continental like that of the 
plateau region, but its extremes are mitigated by low alti- 
tude and the influence of the Great Lakes, Gulf of Mexico, 
and Atlantic Ocean. In the northwest the summers are 
hot and the winters cold, but these characteristics grad- 
ually change toward the southeast, the winters growing 
milder and shorter and the summers hotter and longer. 
The mean annual rainfall increases from northwest to 
southeast, and the annual range and variability of tem- 
perature decrease in the same direction. 

The Polar Regions. — Owing to the prolonged continu- 
ance of daylight and darkness near the poles, the daily 
changes of weather are obliterated and merged with the 
seasonal. There are but two seasons, — a winter of nine 
or ten months, and a summer of two or three months, the 
change from one to the other being very abrupt. Although 
a large amount of insolation is received in summer, it is 
mostly expended in melting the accumulated ice and snow 
and does not raise the temperature much above freezing. 
Consequently the temperatures are persistently low. There 
is no warm season, and snow falls in every month of the year. 

Humidity, cloudiness, and precipitation are greatest in summer. The 
precipitation is probably equivalent to less than ten inches of rainfall, 
but on account of slow melting and evaporation the snow accumulates 
and buries the land under glacial sheets upon which the short summers 
make little impression. Spells of stormy weather, probably cyclonic, 
are frequent at all seasons. Periods of calm are relatively warm in 
summer and intensely cold in winter. Low temperatures with calm, 
dry air are easily endured, but with high damp winds are exceedingly 
dangerous to beasts and men. 



WEATHER AND CLIMATE 



347 




Mountain Cli- 
mate. — Moun- 
tains which rise 
from tropical low- 
lands to a height 
of four or five 
miles exhibit all 
the varieties of 
climate which ex- 
ist between the 
equator and the 
poles, and with 
great regularity. 
As the ■ altitude 
increases, the 
mean annual tem- 
perature falls and 
the yearly range 
increases. The 
annual rainfall in- 
creases to a cer- 
tain height and 
then diminishes. 

At high elevations 
the clearness and dry- 
ness of the air favor 
rapid heating and 
radiation ; hence the 
differences of tem- 
perature between day 
and night and be- 
tween sun and shade 
are great. Travelers 
suffer from the heat 
of the unclouded sun 



348 



THE ATMOSPHERE 



upon their heads, while their feet in contact with snow or earth are in 
danger of freezing. The temperature by day on Mauna Kea, at a 
height of 13,850 feet, rises to 108 and at night sinks to 13 . The 
absolute yearly range in the shade on Mt. Ararat, 17,500 feet high, is 
from 63 to — 40 . 

In free air the changes with elevation are somewhat modified. 
The temperature falls on an average about i° for each 300 feet of 
elevation, but the changes are irregular. Aeronauts find the air com- 
posed of distinct layers or currents between which the changes of tem- 
perature and humidity are rapid. Sometimes there is an iwversion of 
temperatures, or a warmer layer overlying a colder. The following 
records of temperature have been obtained at various heights, chiefly 
from unmanned balloons : — 

53,560 feet —61. 6° in shade 



49,200 
42,000 
38,700 
33,000 
31,500 



A. M 


4 


8 


12 


4 


8 


P. M. 


















































JA 


NUA 


?Y 






























/, 


.--" 


— 


-- 










"-— 





— 


'■/ 












































































S' 


— 


-^ 


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JU 


LY 








/ 








\ 
















/ . 








\ 












/ 
















\ 


\ 






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/ 


















\ 




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FT. t 


BOVE 


THE 


GRO 


ND. 






11OO 


FT. 

































-76° 


-96^ 


-74° 


-58 


-54° 


37.4° 


35.6 


33.8° 


32° 


30.2° 


39.2° 


37.4° 


35.6° 


33.8° 


32° 


30.2° 


28.4° 


26.6° 


24.8° 


23° 



Fig. 310. 



" " — ii u in sun. 

Fig. 309 shows the distri- 
bution of temperature in free 
air, by means of three sec- 
tions of the atmosphere, with 
vertical isotherms. 

The range of temperature 
in free air, both daily and 
seasonal, decreases rapidly 
with elevation. This is dis- 
tinctly observable within a 
short distance above the earth, 
as shown in Fig. 310, which 
shows the average daily range 
at Paris near the surface and 
at the summit of the Eiffel 
Tower. Kite observations 
show that the daily range 
nearly disappears at a height 
of 3300 feet. 



BOOK V. LIFE 

CHAPTER XXX 
PLANT GEOGRAPHY 

Habitable Region. — The habitable region of the earth 
extends through a wide range of elevation and physical 
conditions, from the summits of the loftiest mountains to 
the profoundest depths of the sea. Condors have been seen 
soaring in the cold and rare atmosphere above the highest 
peaks of the Andes, and fishes have been dredged from 
the sea floor where they live in profound darkness and 
under a pressure of five tons of water to the square inch. 
Scarcely any portion of land, air, or water between these 
limits has been found entirely devoid of life. Living 
beings, both plants and animals, in their myriad forms, 
are everywhere dependent upon the physical conditions 
of temperature, humidity, air, and food supply. Their dis- 
tribution is controlled by the most delicate adjustments 
and adaptations of structure, activity, and habit to those 
conditions, and can be explained only by a study of the 
relations existing between relief, climate, and life. 

The Relations of Plants to Soil, Water, Air, and Climate. 
— The distribution of plants over the face of the earth 
depends upon a combination of all the great geographical 
conditions and upon the adaptation of the plant to those 
conditions as they exist in any given locality. Nearly all 
plants sustain certain constant relations to soil, air, and 
climate, and the endless variety of ways in which plants 

349 



350 LIFE 

have adjusted themselves to these relations has determined 
the diversity which we find in the flora or plant population 
of different regions. 

Soil. — Most land plants have roots which ramify exten- 
sively through the soil, and thus furnish an anchorage 
which enables the plant to maintain a fixed position and 
in most cases an upright attitude. Plants also depend 
upon the soil for a supply of water and for materials which 
go to form the ash or incombustible parts. The best soils 
are mixtures of sand, clay, and humus. 

Water. — Most plants absorb water from the soil through 
the roots. Water forms a large part of the raw material 
of plant food. It acts also as a circulating fluid which 
brings in and transports other materials to all parts of 
the plant, somewhat as the blood acts in the animal body. 
The presence of water in the plant cells keeps the stems 
and leaves stiff and turgid. As soon as it evaporates too 
rapidly, the plant wilts. 

Air. — The chief supply of raw materials for plant food 
is derived from the air. Roughly speaking, the combusti- 
ble portion of the plant, or about 75 per cent of its weight 
when dry, is built out of materials supplied from the air. 

Light. — Green plants only are able to combine the 
elements of air, water, and soil,"and to manufacture them 
into food. This process is carried on in the leaves and 
through the agency of sunlight. It can not take place 
in the dark. The air enters the mouths or pores which 
are very numerous on the under side of the leaf. The 
working cells on the upper side of the leaf contain green 
bodies which are able to break up the carbon dioxide of 
the air into its elements, carbon and oxygen. The carbon 
is combined with water to form starch, sugar, and other 
organic products, while the oxygen is given off as waste 



PLANT GEOGRAPHY 35 1 

matter. Thus the green cells of plants act as food facto- 
ries, which are run by the power of sunlight. 

Temperature. — For every plant there is a certain range 
of temperature within which it is able to work. The 
extreme range of the plant kingdom is from about 3 2° to 
122 F. As a rule, any plant will nourish and be vigorous 
in proportion to the duration and height of the tempera- 
ture within the range to which it is adapted. 

The Geographical Distribution of Plants ; Control by- 
Temperature. — The most important condition which con- 
trols the distribution of plants is temperature. While the 
energy necessary for plant growth is supplied by sunlight 
rather than by heat, a proper temperature is one of the 
conditions essential for the accomplishment of all the 
processes of plant life. Hence temperature marks out 
broad zones, within which certain plants may thrive if 
other conditions permit, but outside of which they can not 
exist. Humidity and other secondary conditions determine 
the presence or absence of particular species of plants in 
particular localities within these zones. 

The distance to which any species of plant ?nay extend toward the 
poles, ttp a mountain side, or- into any relatively cold region, depends 
upon the length and average temperature of the growing season. Corn 
requires a higher temperature. and a longer season to mature than 
wheat ; hence it can not be raised in as high latitudes as wheat. Sugar 
cane requires a still higher temperature and longer season than corn ; 
hence it is restricted to regions nearer the equator. The distance to which 
any species of plant may extend toward the equator, or into any relatively 
hot region, depends upon the average temperature of a period of adout 
six weeks at the hottest time of the year. Cotton and sugar cane will 
endure without injury a higher temperature prolonged for six weeks than 
wheat ; hence they can be raised in much warmer climates than wheat. 
Plant Zones. — The plant zones, as determined by tem- 
perature in North America, with some of their character- 
istic forms, are as follows : — 




Fig. 311. — Polar limit of trees, northern Russia. 




Fig. 312. —Coniferous forest, near the timber line, Colorado. 

35 2 



PLANT GEOGRAPHY 



353 



(i) Polar (Fig. 
311). — Lichens, 
mosses, poppy, saxi- 
frages, gentians, and 
willows, all dwarfed, 
tufted, and stunted. 

(2) Cold Temper- 
ate (Fig. 312). — 
Coniferous forests ; 
in the north, spruce 
and fir ; in the south, 
pine, hemlock, cedar, 
redwood, sequoia. 




Fig. 314. —Tropical forest, Mexico. 

DR. THYS. GEOG. 21 



Fig- 313 —Mixed forest, 
North Carolina. 

( 3 ) Warm Tem- 
perate (Fig. 313). — 
Deciduous forests ; 
in the north, beech, 
birch, sugar maple, 
dogwood ; in the 
south, oak, hickory, 
chestnut, walnut, 
sycamore, sassafras, 
tulip tree, hackberry, 
sweet gum. 

(4) Tropical (Fig. 
3 14). — Broad-leaved 
evergreens, long-leaf 



354 



LIFE 



pine, magnolia, live oak, cypress, tupelo, palmetto, palm, 
mahogany, mangrove, yucca, cactus, agave. 

(5) Equatorial (Fig. 315). — Forests characterized by a 
large number of species, the great height of the trees, and 
the number of climbing plants and air plants. 




Fig. 315. — Equatorial forest, Mexico. 

Control by Humidity. — Within the zones of temperature 
the distribution of species and groups of plants is deter- 
mined almost entirely by water supply. According to the 
amount of water which they require, plants may be divided 
into three great classes : (1 ) water plants, (2) drouth plants, 
and (3) intermediate plants. These classes are again 
divided into numerous groups or plant societies, the mem- 
bers of which differ widely as individuals, but are so 
adapted to similar conditions and to one another that they 
are commonly found growing together without serious in- 
terference among themselves. 



PLANT GEOGRAPHY 



355 



Water Plants. —A 

large class of plants 
flourish only in water or 
in very wet soil, (i) 
Floating or submerged 
plants are characterized 
by thin walls through 
which water is absorbed 
by all parts of the plant. 
Roots being unnecessary 
are absent or used for 
anchorage only. The 
plant is supported by the 
water, and has no need 
of stiffness ; hence it is 
soft and flexible. Nu- 
merous species of sea- 





Fig- 317- - Reed swamp. 



Fig. 316. — Plants with floating leaves. 



weed belong to this class, 
•some of which attain such 
dimensions as to rival the 
largest of land plants. 
Bladderworts and duck- 
weeds are common float- 
ing plants in fresh-water 
lakes and ponds. (2) 
Many plants are rooted 
to the soil, -but have sub- 
merged or floating leaves, 
like the pondweeds and 
water lilies. The sub- 
merged leaves are com- 
monly narrow and thread- 
like, and the floating ones 
very broad (Fig. 316). 
(3) Swamp or marsh 
plants are rooted in water 
or very wet soil, while 
their stems and leaves 
are exposed to the air. 



356 



LIFE 




Fig. 318. - Cypress swamp. 




Fig. 319. — The giant cactus, Arizona. 



PLANT GEOGRAPHY 357 

Many societies of them are common in temperate climates : among 
them may be found cat-tail flags, reed grass, sedges, willows, alders, 
tamarack, and cypress. 

Drouth Plants are adapted to thrive in a dry soil and climate. They 
generally have an extensive root system in proportion to the size of the 
plant, a small leaf surface, and a thickened epidermis. Many plants 
survive regular periods of drouth by the disappearance of root, stem, 
and leaves, and the reduction of the individual plant to seeds, bulbs, or 
tubers. The shedding of leaves is a provision against destruction by 
the dry as well as the cold season. The reduction of the leaves to 
threads or needles, as in the pine and other species of coniferous trees, 
and the total absence of leaves, as in the cactus, are efficient means of 
withstanding drouth. The perfection of these adaptations is probably 
found in the melon cactus, in which the whole plant is reduced to a 
spiny, thick-skinned, globular mass. 

Intermediate Plants. — This class of plants adapted to a medium sup- 
ply of water comprises about 80 per cent of all known forms and consti- 
tutes the more common vegetation of temperate regions. The principal 
societies include a great number of species and individuals, but are 
easily recognizable. (1) Arctic-Alpine carpets are found in the polar 
regions and at high elevations where the low temperature and short 
season prevent tall growth. The northern borders of Eurasia and 
North America are occupied by the tundras, where the ground is per- 
petually frozen to a great depth, and in the short summer thaws out 
only on the surface. These are covered chiefly with mosses and lichens. 
In more favorable localities, where higher plants occur, the vegetation has 
a remarkably fresh and green appearance, the growth is very rapid, and 
the flowers are of large size and bright colors. The shrubby plants 
spread out close to the ground and the whole forms a low, soft, dense, 
brilliant covering, appropriately called a carpet. (2) Prairies, in the 
central United States, are extensive plains having a deep, rich, humus 
soil with a moderate supply of water. They are exposed to irregular 
drouths and to dry, hot winds unfavorable to tree growth. They are 
covered with a luxuriant growth of grass and herbs, among which sun- 
flowers, golden rods, and compass plants are conspicuous. Some 
timber occurs along the streams. Similar tracts are called steppes in 
southern Eurasia, and llanos and pampas in South America. (3) De- 
ciduous forests consist of trees that shed their leaves. A comparison 
of the maps on pp. 328 and 358 shows that forests are characteristic of 



PLANT GEOGRAPHY 359 

regions of large rainfall. Practically no forests occur in regions of less 
than twenty inches of rainfall except the coniferous forests in regions 
of low temperature where evaporation is slow. In deciduous forests 
the thin, broad, horizontal leaves give the largest possible surface 
exposure to air and light and have great working power while the 
growing season lasts. (4) Broad-leaved evergreen forests occur in 
the trade-wind belts of heavy rainfall, high temperature, and rich soil, 
and are especially luxuriant in the Amazon basin. The new leaves con- 
tinually appear before the old ones are shed. The growth is very dense 
in layer above layer, mosses, herbs, shrubs, and trees, and all covered 
and overrun by air plants and climbing plants. The trees are tall, 
straight, and without limbs except at the very top. The number of 
species is astonishing. On 60 square miles in Brazil 3000 species of 
plants, including 400 species of trees, have been counted. 

Control of Distribution by Methods of Migration. — While 
individual plants are usually anchored to one spot, plant 
species migrate as truly as birds, and some, have traveled 
around the world. If the conditions are favorable, plants 
are continually spreading so that each generation occupies 
more territory than the preceding. 

The methods of plant migration are very numerous. (1) The seeds 
of many plants are scattered by the wind, and many seeds are provided 
with wings, hairs, or down for this purpose. The maple, poplar, cotton- 
wood, sycamore, dandelion, milkweed, and thistle are familiar examples 
of plants whose seeds are specially adapted for transportation by the 
wind. Hence they travel more rapidly and establish themselves in a 
new field more quickly than heavy-seeded plants. (2) Many seeds are 
provided with hooks by which they attach themselves to animals and 
the clothing of men. These include what are popularly known as ticks, 
stickers, beggar's lice, burdocks, and cockleburs, and many other varie- 
ties. (3) The seeds of edible fruits and berries are carried away by 
birds and other animals. (4) Some plants, like the touch-me-not. have 
explosive seed pods which burst when ripe and throw the seeds to some 
distance. (5) Seeds may be floated by rivers and ocean currents to 
great distances. Trees are apt to extend farther into new territory 
along streams, and the shell of the cocoanut enables it to survive a long 
voyage in salt water. (6) Man is, often unintentionally, a most effi- 
cient disseminator of plants. Besides the various cultivated plants 



360 LIFE 

which he has carried to all parts of the world, many undesirable ones 
have accompanied him. Most of our common weeds are robust and 
vigorous foreign plants which have extended themselves along lines of 
human travel, as the Russian thistle, French corn cockle, and Italian 
mustard. 

Control of Distribution by Barriers. — Migration is not 
unlimited, because there are barriers which most plants 
can not cross. The most efficient are oceans, mountain 
ranges, deserts, and regions already occupied by luxuriant 
vegetation. The plants of North America and Eurasia are 
much more alike than those of South America and Africa, 
because they have not been separated by such wide ocean 
barriers. The plants of Florida and Canada are more 
alike than those of Florida and Cuba, because the Florida 
strait, swept by the Gulf Stream, is an effectual barrier 
between these two regions. 

The Struggle for Existence. — It requires but slight obser- 
vation to see that in nature very few plants find the best 
conditions for their growth. The difference between a 
neglected roadside and a well-kept garden illustrates this. 
By the roadside a multitude of grasses, weeds, and bushes 
struggle with one another for possession of the ground. 
The weaker are finally crowded out by the stronger, but not 
one has all the light, air, water, and soil to itself. In the 
garden a few selected plants are given all the room they 
need, and are fed, watered, and protected, while rivals and 
enemies are carefully destroyed. In nature each plant 
must maintain itself, as best it can, against the rivalry of 
other individuals of the same kind, against plants of other 
kinds which are adapted to similar conditions, and against 
numerous insects, birds, and burrowing or grazing animals. 
Every blade of grass in the meadow and every tree in the 
forest is more or less interfered with by its neighbors and 
gets only such a share of light air, and water as it is able 



PLANT GEOGRAPHY 36 1 

to seize in spite of them. Most plants multiply so rapidly 
that all their offspring could not possibly find room on the 
earth. A single tobacco plant produces 360,000 seeds in 
a year. If every seed produced a plant, in a few years the 
whole surface of the dry land would be covered with them. 

Changes of Environment. — Not only does the natural 
spread of plants carry them into new conditions, but the 
conditions in any region or locality are subject to change. 
Lakes and swamps dry up, are filled with sediment, or are 
drained. Areas usually dry are sometimes flooded. The 
growth of tall and rank vegetation cuts off the supply of 
light for the lower forms. The clearing of forests subjects 
the shade-loving plants which grew beneath them to new 
trials. Widespread changes of climate have sometimes 
involved a large part of a continent, as that from wet to 
dry in the Great Basin and from warm to cold during the 
glacial period in North America and Europe. If such 
changes are rapid, all or most of the plants are destroyed ; 
if they are sufficiently slow, the species of plants may sur- 
vive by migration to other regions. 

Adaptation. — Whether plants remain at home or under- 
take migration, they are subjected to a constant struggle 
for existence, by the competition of their neighbors and 
by the pressure of new conditions. They are all subject 
to the law of heredity, by which new plants resemble in a 
general way the parent stock. They are also subject to 
the law of variation, by which every individual plant dif- 
fers more or less from all others. Among all the millions 
of slight variations some are advantageous to the plant 
and some are the reverse. Those plants which are en- 
dowed with some advantage of form, structure, or habit, 
are, to that extent, better able to compete with their fel- 
lows, and with changed conditions. Such plants are 



362 



LIFE 



more likely to survive, to mature seed, and to produce 
numerous offspring, to which they transmit their own 
peculiarities. This goes on from generation to genera- 
tion, advantageous peculiarities become permanent, and 
the successive generations become so different from their 
ancestors and relatives as to constitute new varieties and 
species. This process is called adaptation or the survival 
of the fittest, which means only that no two plants are 
exactly alike, and that some, being better fitted to contend 
with unfavorable conditions, necessarily survive and mul- 
tiply faster than those more poorly equipped. By such 
processes the hundreds of thousands of species of plants 
known to exist in the world have developed from a few 
original forms. Thousands of species which once flour- 
ished have become extinct, while new varieties and species 
are still being produced. 

Influence of Past History. — From all these considera- 
tions it is evident that the distribution of plants in any 
region has been determined not only by the present con- 
ditions of the region, but also by its past history. The 
greatest contrast exists between the plants of the northern 
hemisphere and those of the southern. Very few species 
are found in both except those which have been carried 
by man. The equatorial region has been the cradle and 
nursery of plant life, in which species have originated, and 
from which they have migrated north and south. The 
ground there being fully occupied, the luxuriant vegetation 
forms a barrier practically impassable from one side to the 
other. The variety of vegetation in any country depends 
largely upon its age, that is, the time which has passed 
since it was last covered by water or ice. 

Brazil is a very old country, and the number of species growing 
there is very large. Northern Europe is a very young country. The 



PLANT GEOGRAPHY 363 

ice sheet of the glacial period swept it clean of its pre-glacial flora, and 
there has not been time since for many species to establish themselves ; 
their present number is not half • that in Brazil. In North America the 
glacial period forced northern plants southward, where many still remain. 
The majority of plants in the region of glacial drift are recent immi- 
grants. Some, like the willows and maples, betray their northern ori- 
gin by hastening to produce their flowers and seed in early spring ; 
others, like the asters and golden rods, show their southern origin by 
taking the whole summer to mature, and producing flowers and seeds 
only in the autumn. The north-south mountain ranges of North 
America have not been barriers to migration so much as roads along 
which plants have traveled southward, and then descended into the 
plains. The east-west mountain ranges of Europe have been effectual 
barriers. During the glacial period few northern plants were able to 
escape destruction by migrating over the mountains, and since its close 
few have been able to reach northern Europe from the south ; hence 
that region has fewer species than North America. 



CHAPTER XXXI 

ANIMAL GEOGRAPHY 

Relations of Animals to Climate and Plants. — The tis- 
sues of the animal body are able to maintain life and ac- 
tivity only under definite conditions of temperature ; but 
most animals have the power to maintain within them- 
selves the proper temperature, whether it be higher or 
lower than that of the surrounding medium. Hence they 
are less dependent upon climatic conditions than plants. 
Animals require water and a constant supply of oxygen 
from the air. The most important relation which animals 
sustain to other geographical conditions is their depend- 
ence upon plants for food. No animal has the ability to 
live upon mineral food alone, or to manufacture organic 
food from the raw materials of earth, water, and air. This 
can be done only by the green cells of plants, and upon 
such organic food all animal life is absolutely dependent ; 
for flesh-eating animals feed on plant-eating animals. Thus 
the distribution of animals is influenced either directly or 
indirectly by the same conditions as that of plants. The 
laws of heredity and variation prevail among animals, and 
they are subject to the rivalry and competition of their 
neighbors to a degree proportional to their complex 
organization and wide range of activity. The struggle 
for existence is severe and destructive to untold millions 
of individuals, and only those survive which are able to 
develop some peculiar power or resource which gives them 
an advantage over their fellows. To procure food and to 
defend itself and its offspring from enemies, are the chief 
problems which every animal must face. 

364 



ANIMAL GEOGRAPHY 365 

Origin and Distribution of Animals. — The millions of 
animal forms which now people the earth have descended 
from a few parent forms which were probably unlike 
any now in existence. Every animal has its origin far 
back in the geologic past. In many cases a long series of 
fossil forms are known which connect living animals with 
their remote ancestors. The line of descent of the modern 
horse has been traced back through ancestral forms, more 
and more unlike a horse, to a five-toed animal of the size 
of a fox. To account for the present distribution of any 
species of animal, it is necessary to consider two ques- 
tions : where did it originate ? and how did it reach its 
present home ? The first question is answered, if at all, 
by a study of the rocks which contain the fossil remains 
of extinct forms, and a discovery of the locality and period 
in which the animal in question or its ancestors first ap- 
peared. The second question is answered, if at all, by a 
study of the methods of migration which the animal pos- 
sesses, and of the barriers or bridges which have existed 
between its birthplace and its present home. 

Migration of Animals. — Animals have much greater 
facilities for migration than plants, yet they are restricted 
in their wanderings by the same great barriers of ocean, 
desert, and mountain. The effect of barriers and the vary- 
ing ability of different animals to surmount them may be 
illustrated by a case which might occur among the domes- 
tic animals of a farm. Suppose a farmer has four fields 
arranged as in Fig. 320. B is separated from A by a wide, 
close hedge; C by a high, strong fence; D by a pond. If 
the animals named are placed in A, which would be likely 
to get into each of the other fields first ? The swine might 
work through the hedge into B, the ducks would quickly 
swim across the pond to D, followed perhaps by the swine, 



366 



LIFE 



HORSES 




FOWLS 


CATTLE 








A 


DUCKS 


SHEEP 




PIGEONS 


SWINE 






■■ill 

HEDGE 


FENCE 


SSSEE 




POND 


B 


C 


D 



Fig 320. 



cattle, and horses. Some horses might jump the fence into 
C, and some fowls would be able to fly over it. The sheep 

would remain in A, 
while the pigeons would 
be unrestrained by any 
of the barriers. The 
actual problem of dis- 
tribution is complicated 
by the fact that as ani- 
mals migrate into new 
conditions of climate 
and food supply, and 
meet new rivals and 
enemies, those which 
survive usually do so by 
gradual adaptation to 
their changed environment, and thus through thousands 
of generations become more and more unlike their ances- 
tors and one another. 

Areas of Distribution. — Each species of. animal occupies 
a certain territory which is called its area of distribution. 
The areas of distribution of different species usually over- 
lap ; when the boundaries are sharply defined, they are 
formed by natural barriers, such as a river, strait, moun- 
tain, forest, or treeless plain. 

Of all animals birds are the most widely distributed. The fishhawk 
and barn owl range over nearly the whole world. Crows, swallows, 
doves, grouse, hawks, owls, snipe, herons, ducks, gulls, petrels, and peli- 
cans occur in all parts of the world. Humming birds range from Cape 
Horn to Alaska, and from sea level to the snow line. Among mammals 
the only family with great powers of flight, the bats, have as wide a dis- 
tribution as birds. Many similar cases are found among butterflies and 
beetles. The Bengal tiger ranges from the hot, damp jungles of south- 
ern India over the loftiest mountains on the globe to the dry steppes of 



ANIMAL GEOGRAPHY 



367 



Siberia, yet he has never been able to cross the forty miles of water 
between India and Ceylon. Equally remarkable examples are found 
of species which are restricted to a single small area. The gorilla is 
confined to the equatorial forests of west Africa, the aye-aye to Mada- 
gascar, and a peculiar lizard to one or two small islands off the coast of 
New Zealand. One species of humming bird has been found only in 
the crater of Chimborazo, and a certain species of fish in a single small 
lake in Scotland. In the Hawaiian Islands each valley, often each side 
of a valley, and sometimes each ridge and peak, has its own peculiar 
species of snails. Fresh-water fishes and land snails generally have the 
smallest range. Many instances of restricted range may be explained 
by the fact that the species is either a new one which has not yet had 
time to spread, or an old one which once occupied a wider area and is 
now nearly extinct. 

The Zoological Realms and Regions. — As shown by Fig. 
321, the land surface of the earth is divided according to its 




Fig. 321. — Animal realms and regions. 

living inhabitants into three realms, which are very unequal 
in area, but differ widely in their flora and fauna. They 
are: (1) the Australian Realm, comprising Australia, Tas- 
mania, New Zealand, New Guinea, and other neighboring: 
islands ; (2) the South American Realm, comprising South 



368 LIFE 

and Central America and the West Indies; and (3) the 
Northern Realm, comprising Eurasia, Africa, and most of 
North America. It is to be noticed that the southern con- 
tinents, which are widely separated from one another by 
the oceans, belong each to a different realm, while the 
northern continents, which form an almost continuous land 
mass, are all in the same realm. 

The Australian Realm presents the most peculiar assem- 
blage of plants and animals in the world. The great body 
of it is desert or semi-arid and unfavorable for the develop- 
ment of the higher forms of life. 

Its mammals, with few exceptions, belong to two small and unique 
groups, the egg layers (jnonotremes) and the pouched animals (marsu- 
pials). The former is represented by the duckbill, an aquatic, burrow- 
ing animal of the general form, size, and fur of a muskrat, but having 
webbed feet and a broad bill like a duck. It lays eggs and suckles its 
young. The pouched animals, represented by the kangaroo, have a 
pouch, formed by a fold of the skin of the abdomen, in which the young 
are carried for some time after birth. In form and habits they are very 
diversified, some resembling wolves, others weasels, squirrels, rabbits, 
moles, rats, and mice. The largest is the kangaroo, which, when sitting 
upon its hind legs and powerful tail, is as tall as a man. The only 
pouched animals found outside of Australia are the opossums of Amer- 
ica. The only other Australian mammals are rats, mice, bats, and a 
single species each of pig and dog. The birds are almost as peculiar 
as the mammals ; they include paradise birds, lyre birds, cockatoos, 
many species of parrots and pigeons, the large and almost wingless 
kiwis, emus, and cassowaries, and the brush turkeys, which build large 
mounds of leaves and brush wherein their eggs are laid and hatched 
by the heat produced as the pile ferments. 

The plant life of the Australian Realm is as odd as its animals. 
Many of the trees are leafless or have grasslike leaves. The gigantic 
eucalyptus or gum trees have leaves which stand with their edges to the 
sky. Many new plants and animals have been introduced by man. 

The animals of New Zealand are so remarkable as to distinguish it 
from the rest of the Australian Realm. Although a large continental 
island, it has no mammals except two kinds of bats and one species of 



ANIMAL GEOGRAPHY 



369 




Fig. 322. — Australian animals. 

rat. It is characterized by several species of gigantic wingless or flight- 
less birds, now nearly or quite extinct. Their nearest relatives are 
found in the ostrich of Africa and the rhea of South America. 

The fossil remains of the earliest ancestors of the pouched animals 
are found in Eurasia, and it is practically certain that they migrated 
thence to Australia at a very remote period when that continent had 
a land connection with Asia. By a subsidence of the land bridge they 
were cut off from the rest of the world, and the access of higher and 
more vigorous forms was prevented. The absence of large birds or beasts 
of prey relieved them from their most formidable enemies, and they have 
been permitted to multiply in peace and safety- Australia is a sort of 
biological museum in which forms which long ago nourished and were 
destroyed on other continents have been entrapped and preserved. The 
realm is separated from Asia by a series of deep straits called " Wal- 
lace's line." Between the islands of Bali and Lombok the strait is 
only fifteen miles wide, yet a large proportion of the animals, including 
even half of the birds, are different upon opposite sides of it. 

DR. PHYS. GEOG. 22 



370 LIFE 

The South American Realm is bounded upon all sides 
by wide and deep oceans except on the north, where it 
is connected with the Northern Realm. It is a country of 
extensive tropical forests and open grassy plains, admira- 
bly adapted for the support of abundant life. The forests 
are the most extensive and luxuriant in the world, covering 
the low plains and extending up the mountain sides to a 
height of 8000 or 9000. feet. The open grass lands of 
Venezuela and northern Brazil alternate with the forest, 
while the pampas of the south are a vast sea of grass. As 
might be expected, this realm is surpassingly rich in nearly 
all forms of life. 

It contains representatives of more than half of all the vertebrate 
families in the world, and more than one fourth of these families occur 
nowhere else. Its most characteristic animals are the edentates, tooth- 
less and armored forms represented by the armadillo, an animal of the 
size and habits of a small pig, but covered with bony plates. When 
attacked it rolls itself into a ball which is completely covered by the 
shell. In former times similar animals of gigantic size, having a shell 
six or eiglit feet in diameter, were numerous. To the same group 
belong the sloths and anteaters. In the hot forest regions are the 
prehensile-tailed monkeys and marmosets, both peculiar to this realm. 
There are a few deer, but no other hoofed animals except the peccary, 
allied to the pig, and the tapir, allied to the elephant. The rodents, 
or gnawers, are represented by the chinchilla, the guinea pig, and the 
capybara, the largest of its order. Vampire and leaf-nosed bats are 
numerous. The camel tribe is represented by four peculiar species, the 
llama of the high Andes, the only native beast of burden ; and the 
alpaca, vicuna, and guanaco, valuable for their wool. The principal 
beasts of prey are the puma, or panther, and the fierce and powerful 
jaguar, which successfully attacks horses and oxen. In birds this realm 
is even richer than in mammals, and five sixths of them do not occur 
elsewhere. There are nearly 400 species of humming birds. Other 
characteristic birds are sugar birds, tanagers, chatterers, tree creepers, 
parrots, toucans, curassows, trumpeters, umbrella birds, sun bitterns ; 
condors, the largest of flying birds ; and the rhea, or American ostrich. 
Among reptiles are the boa constrictor and anaconda, the largest of 



ANIMAL GEOGRAPHY 



371 



serpents ; turtles, alligators, crocodiles, and numerous lizards. In fresh- 
water fishes and insects the realm is rich almost beyond description. 

One of the strangest peculiarities of the realm is the fact that 
although the pampas form a paradise for grazing animals, there are no 
native cattle, horses, sheep, goats, or antelopes, the vast herds which 
now exist there having been introduced by man from Europe. 

Among the thousands of native plants peculiar to the realm are many 
palms, the milk or cow tree, melon tree, custard apple, evergreen 




ARMADILLOS 



Fig- 323. — South American animals. 



beech, Brazil nut, araucaria, Paraguay tea, Victoria regia, pampas grass, 
and those which yield logwood, quinine, guava, chocolate, rubber, 
spices, and varnishes. The potato, tobacco, cayenne pepper and Indian 
corn probably originated in South America. 

The peculiarities of South American forms of life point to long 
periods during which the continent was separated from the rest of the 
world, and to a gradual increase in land area and in diversity of relief 
and climate. These conditions were favorable to the development of 
species in extraordinary numbers and variety. The isolation was inter- 



372 LIFE 

rupted at various times by temporary land connection with North 
America, which furnished opportunities for invasion by many northern 
forms. 

The Northern Realm, comprising four fifths of the land 
surface of the globe, is characterized by a general simi- 
larity of plant and animal forms, as contrasted with those 
of the Australian and South American realms, but it is 
divided into several more or less distinct regions. 

The African Region, including Africa south of the 
Sahara, and the island of Madagascar, is not far behind 
South America in number, variety, and peculiarity of 
species. In the east it is a moderately elevated plateau 
with a hot and rather dry climate, occupied by open grassy 
plains and hills with patches of forest. In the west 
the dense forest resembles that of South America. In the 
south semi-arid and desert conditions prevail. 

In this life region are found the manlike, tailless apes, the gorilla 
and chimpanzee ; baboons and lemurs ; the elephant, rhinoceros, and 
hippopotamus, the largest of land animals ; the giraffe, zebra, quagga, 
lion, leopard, jackal, hyena, and about eighty species of antelopes. It is 
equally remarkable for the absence of tigers, wolves, foxes, bears, wild 
oxen, deer, sheep, goats, camels, moles, and true pigs. It abounds in 
eagles, vultures, and guinea fowls, and has the peculiar serpent-eating 
secretary bird and the ostrich. Serpents are numerous, including vipers 
and puff adders. It is also the home of the chameleon. Two thirds 
of its mammals and three fifths of its birds do not occur elsewhere. 

Madagascar bears the same relation to Africa that New Zealand does 
to Australia, but possesses a richer and more remarkable fauna than 
New Zealand. It is characterized especially by lemurs, bats, civet cats, 
and a great variety of birds, but most of the groups abundant in Africa 
are wholly wanting. 

The plants of the African Region include the baobab, cotton tree, 
oil, wine, and screw palms, banana, plantain, yam, manioc, mangrove, 
breadfruit, coffee, aloe, tree spurges and heaths, prickly acacias, lilies, 
orchids, pelargoniums, and the strange welwitschia. 

The African Region has for a long period been separated from north- 
ern lands by the Sahara, which is almost as efficient a barrier to migra- 



ANIMAL GEOGRAPHY 



373 




Fig. 324. — African animals. 



tion as a sea of equal width. In early times South Africa and Mada- 
gascar were united with each other, and possibly with India, by a land 
area in the present Indian Ocean. Later, Africa was separated from 
Madagascar and united with Eurasia so as to permit an influx of higher 
and larger animals from the east and north. 

The Oriental Region includes Asia south of the Hima- 
laya and Sulaiman mountains, together with neighboring 
islands. Its area is small, but its surface is very diversi- 
fied. The mountains form a less efficient barrier than the 
sea or desert ; consequently this region is less peculiar 
than the African, with which it has many features in 
common. 



374 



LIFE 



It is remarkable for its orangs, chimpanzees, and an abundance of 
monkeys, gibbons, and lemurs. It is especially rich in the cat tribe, 
of which the tiger is easily chief, with the lion, leopard, and panther not 
much inferior. There are thirty species of bats, and fifty each of mice 
and squirrels. The elephant, rhinoceros, bear, tapir, wild boar, and 
wild cattle abound. Of birds it has contributed the pea fowl, pheasant, 
and jungle cock, from which the common domestic fowl is derived. 
Tailor birds, thrushes, woodpeckers, cuckoos, and hornbills are com- 




Fig- 325. — Oriental animals. 

mon. Among reptiles, crocodiles and venomous serpents are very 
abundant. About half of the mammals and birds are peculiar. In 
number, variety, and beauty of birds, butterflies, and beetles this region 
is unrivaled. 

Among its most useful plants are the bamboo, banyan, sago and 
areca palms, camphor tree, teak, gutta-percha tree, balsam, incense, 
cinnamon, nutmeg, pepper, clove, and mangosteen. 

The ancestors of most of the characteristic forms of the Oriental 
Region lived in Europe and northern Asia at a time when a tropical 
climate extended far toward the pole ; the lofty plateaus and mountains 



ANIMAL GEOGRAPHY 



375 



of central Asia had not yet been uplifted, and substantially one fauna 
ranged over the whole of Eurasia. The elevation of the highlands, 
the northward extension of the Siberian plain, and the change to a 
more severe climate will account for the diversity and division now 
found in that continent. 




Fig. 326. — Eurasian animals. 



The Eurasian Region contains the whole of Europe, 
Africa north of the Sahara, and the whole of Asia except 
the Oriental Region and the southern part of Arabia. 
Extending from Iceland on the west to Japan on the east, 



376 LIFE 

it comprises all the north temperate portions of the east- 
ern hemisphere, yet the majority of its plants and animals 
are identical throughout. It contains very few peculiar 
forms, but has been a center of development for a large 
number which have migrated from it to other regions. In 
comparatively recent times, the elephant, hippopotamus, 
rhinoceros, and lion were as abundant in Europe as they 
now are in Africa, while the mammoth or hairy elephant of 
Siberia, recently extinct, is thought to be the immediate 
ancestor of the Indian elephant. 

Almost the entire family of moles is confined to this region, as is 
also the badger. Of hoofed animals the camels have their native 
home in central and western Asia. There are many peculiar species 
of lynxes, badgers, deer, oxen,- sheep, goats, and antelopes, among them 
the yak of Tibet and the chamois of the Alps. There are also peculiar 
forms of rodents. Wild horses occur in central Asia, several species 
of wild cattle in Europe, and reindeer in the north. Elks, bears, and 
wolves are widely distributed. Numerous species of birds are charac- 
teristic, among which are crows, finches, larks, magpies, choughs, 
grouse, pheasants, and jays. Many of the birds which belong truly to 
this region migrate southward in winter. Snakes and lizards are com- 
paratively scarce, but toads, frogs, and newts abound. The great simi- 
larity between the flora and fauna of Europe and North Africa indicates 
that land connections have recently existed across the Mediterranean 
from Africa to Spain and Italy. 

The most common and widespread trees are the oak, fir, beech, 
birch, elm, pine, poplar, maple, and ash, while in the south the chestnut, 
plane, mulberry, olive, cork oak, lime, pomegranate, orange, laurel, 
myrtle, and fig abound. In north Africa, the date palm, doum palm, 
aloe, oleander, papyrus, and lotus are added. The tea plant and paper 
mulberry are natives of eastern Asia. 

The North American Region consists of North America 
north of central Mexico. It possesses a great variety of 
climate and surface, but it is widest toward the north, and 
suddenly narrows in its warmest portion. A large extent 
of its interior is subject to great extremes of temperature, 



ANIMAL GEOGRAPHY 



377 



and much of it is semi-arid or desert. It has recently 
been subjected to severe and widespread glaciation, which 
must have destroyed many of its inhabitants. It is not 
strange that its fauna is less rich and varied than that of 
the Eurasian region, which it most resembles. 







Fig. 327. — Worth American animals. 

Among its peculiar species are the star-nosed moles, weasels, and 
skunks, the prong-horned antelope, the bighorn or Rocky Moun- 
tain sheep, musk ox, muskrat, pouched rat, jumping mouse, vesper 
mouse, prairie dog, raccoon, tree porcupine, sewellel, opossum, sala- 
mander, rattlesnake, and horned toad. A large proportion of its birds 
are migratory. It is the birthplace of the horse and the camel, 
although both were extinct there at the time of settlement by Euro- 
peans. It possesses so many forms in common with Eurasia that 



378 LIFE 

the two regions are sometimes united under the name of the Holarctic 
Region. The bison, reindeer, grizzly bear, moose, Arctic fox, lynx, 
wolf, marten, and beaver are characteristic and common to both, and 
point to a time when the two continents were broadly connected, prob- 
ably in the region of Bering Strait. 

The Fauna of Islands. — Continental islands generally 
possess a fauna similar to that of the neighboring conti- 
nent, but much poorer in number and variety of forms. 
Oceanic islands are usually very poor in species of both 
plants and animals. Mammals are often entirely wanting. 
Birds and insects, having the power of flight, and reptiles, 
which in some unknown way are able to cross wide spaces 
of ocean, are more numerous. The conditions upon any 
small island or group of islands are so simple and uniform 
as to be unfavorable to the development of diversity 
among its inhabitants. 

Summary. — All the evidence points to the conclusion 
that the land masses of the northern hemisphere are of 
great antiquity, and have been the birthplace of all the 
higher forms of life. In them are found the fossil remains 
of the ancestors of the animals still living there, and in 
older rocks the forerunners of those which now inhabit 
the southern hemisphere. As the southern lands were 
from time to time temporarily connected with the north- 
ern, successive waves of life flowed into them. There 
have been many minor movements back and forth, but in 
general the newer and higher forms have developed in the 
north and have forced the older and less competent to 
emigrate. Hence the faunas of the different regions be- 
come more and more unlike toward the southern extremi- 
ties of the land, where primitive forms still survive. 
They are now most numerous in Australia, because there 
they have been longest cut off and protected from inva- 



ANIMAL GEOGRAPHY 379 

sion. The number and diversity of species and the den- 
sity of animal population in each region are largely 
determined by the extent and variety of favorable condi- 
tions which prevail there. This law is modified by the 
growth or age of the region, by the efficiency of the bar- 
riers which surround it, and by the opportunities which 
have existed for foreign invasions. South America has 
been most fortunate in a combination of all these circum- 
stances ; hence the surpassing richness of its life. Africa 
and the Oriental Region are not far behind, while Australia 
is distinctly poor. At the extreme of poverty stand the 
small and isolated oceanic islands. Existing forms are 
not always found in regions best adapted to them, but in 
regions where their ancestors lived or to which they have 
been able to migrate from their ancestral home. 

Life in the Sea. — No part of the ocean is devoid of life, 
and most of it is more densely populated than the land. 
The temperature of sea water varies as much between the 
surface and bottom as between the equator and the poles ; 
consequently the distribution of life is in zones more strongly 
contrasted in a vertical than in a horizontal direction. 

Shallow Waters. — The shallow waters, less than 600 feet 
in depth, are penetrated by the light and heat of the sun 
and are most populous. Their populousness is also due to 
the deposit of mud from the rivers, which furnishes suitable 
soil and food for plants and animals. This zone is quite 
definitely bounded by the "mud line" or outer limit of 
continental deposits. In this zone a luxuriant growth and 
variety of seaweeds afford food and protection for a still 
more numerous and varied animal life. 

A sand bottom is inhabited by skates, soles, and flatfish, which are 
colored upon the upper side like the sand. A rock bottom supports 
fixed animals, such as barnacles, worms, polyps, sea anemones, sponges, 



3§o 



LIFE 



corals, and sea squirts, and affords suitable conditions for creeping or 
crawling animals like sea urchins, starfish, periwinkles, sea snails, lob- 
sters, prawns, and crabs. The shallow equatorial waters in which the 
temperature never falls below 68° F. are the home of the coral polyp, 
where extensive reefs afford grazing ground for innumerable forms of life. 




Fig. 328. — Marine mammals. 

Surface Water. — The surface waters everywhere abound 
in free floating or swimming organisms. The largest sea- 
weeds float freely without attachment to the soil and drift 
with the winds and currents. The surface waters teem 
with myriads of small animals which like the jellyfish rise at 
night and sink by day. The vast area, the abundance of 
mineral food in solution, the volume of sunlight, and the 



ANIMAL GEOGRAPHY 381 

uniformity of temperature are favorable to the growth of 
microscopic plants in such numbers as to furnish an inex- 
haustible supply of food for the animals of the sea. 

Among large, free-swimming animals, fishes are the most abundant ; 
but the great marine mammals, whales, porpoises, and dolphins, which 
can not breathe under water and resemble fish only in form, are 
included. Other mammals, like the seal, walrus, sea elephant, sea 
lion, sea bear, and manatee, resort to the land a part of the time for 
food, rest, protection, and breeding. Many fish, like the cod, herring, 
mackerel, and salmon, periodically visit shallow water or ascend rivers 
for the purpose of spawning. 




Fig. 329. —Deep-sea fish, with lanterns. 

Deep Waters. — Fishes as well as invertebrates live near 
or upon the floor of the ocean at all depths. They prey 
upon one another or obtain food from the organic matter 
contained in the bottom mud or ooze, the supply of which 
is maintained by the sinking of plants and animals from 
the surface. 

Some deep-sea animals are blind and some have large eyes. Many 
have limbs or antenna; of extraordinary length, which they use as feelers. 
Others are provided with organs which produce a phosphorescent light 
and serve as lanterns. Phosphorescence is also characteristic of the 
minute freely floating organisms which abound in the surface waters of 



382 LIFE 

some parts of the ocean, and at night cause the water to glow with a 
soft radiance. 

The warm tropical waters are characterized by animals which secrete 
lime shells, while in the cold polar waters lime-secreting organisms are 
few. 

Temperature is the chief barrier to migration in the sea, but the 
polar waters of the two hemispheres are connected along the bottom by 
a continuous layer of water near the freezing point, and the marked 
similarity of the fauna of the Arctic and Antarctic oceans may be due 
to the free communication between them. 



CHAPTER XXXII 
THE GEOGRAPHY OF MAN 

"And God said unto them, Be fruitful, and multiply, and replenish 
the earth, and subdue it." — Genesis i. 28. 

The Ascent of Man. — There is no reason to doubt that 
man, like other animals, has descended from ancestors who 
were unlike himself. The close physical resemblance be- 
tween men, apes, monkeys, and lemurs, indicates that they 
are his nearest relatives, and that the common ancestor 
from which they all sprang was an animal now extinct. 
The most important differences between men and the higher 
apes are : ( 1 ) the ability to stand and to walk upright, and 
the adaptation of the feet and limbs 'for locomotion only; 
(2) the greater perfection of the arms and hands, which are 
left free for use solely as organs for grasping, holding, and 
performing delicate and complex movements ; (3) the in- 
creased capacity of the skull and the greater size and com- 
plexity of the brain, which accompany and render possible 
the enormously greater development of the mental powers. 
The sutures of the brain case do not unite for twenty years 
or more, which enables the brain to continue to grow during 
that period. Thus the period of immaturity during which 
the child can be trained and educated is prolonged. The 
human animal only is highly educable and guided by in- 
telligence and reason. 

Stages of Culture and their Relation to Food Supply. — 
Men, like other animals, are dependent upon their environ- 
ment for food, and the stage of culture or degree of ap- 

383 



384 LIFE 

proach toward civilization attained by any people depends 
largely upon the manner in which they provide for them- 
selves the means of subsistence. The earliest men lived 
upon fruits, roots, seeds, and tubers found growing wild, 
and upon reptiles, insects, worms, and other vermin which 
they could capture. The history of the race has been one 
of slow progress from this lowest stage of savagery through 
barbarism to civilization. The discovery of the use of fire 
and the manufacture of axes, spearheads, knives, and other 
implements and weapons from stone enabled them to be- 
come hunters and fishermen, and added to their resources 
a relatively abundant supply of cooked meat. The inven- 
tion of the bow and arrow was of prime importance, the 
power, range, and accuracy of this weapon giving its 
possessors a decided superiority in war and the chase. 
The invention of the art of making pottery from clay 
added to the conveniences of domestic life facilities for 
the storage of liquids and for cooking by boiling. 

The Domestication of Animals. — As long as men de- 
pend for food supply upon native plants and the killing 
of wild animals, their means of subsistence is irregular 
and uncertain. A country can support only a sparse popu- 
lation, and these are necessarily scattered in small com- 
panies. Men can not rise out of savagery until they have 
placed their food supply upon an artificial basis. This was 
accomplished at a very early period in Eurasia, where 
men learned to keep cattle, horses, sheep, and goats under 
protection and control, and to obtain from them a constant 
supply of food in the form of milk and flesh. Animals 
capable of being thus domesticated were entirely absent 
from Australia, and nearly so from America ; hence the 
aboriginal inhabitants of these continents, with few excep- 
tions, did not rise above the savage state. 



THE GEOGRAPHY OF MAN 385 

The Domestication of Plants. — The domestication of 
animals affords only a partial escape from savagery. 
Among a purely pastoral people the flocks and herds 
depend upon natural pasturage, found in semi-arid steppes 
or prairies, like those of western Asia, eastern Europe, 
and central North America, and must be led or driven 
about from place to place. Under such conditions men 
can not occupy fixed habitations, or gather in large num- 
bers in one place. The domestication of plants is a 
more important step toward civilization than that of ani- 
mals. The fruits most valuable for food supply are con- 
fined to the tropical regions. The fig, date palm, cocoa 
palm, breadfruit, banana, and plantain have formed the 
staple subsistence of millions of people. Of still greater 
value are roots and tubers like the potato, manioc, yam, 
and sweet potato. The most valuable of all are the cereal 
grains. In the Old World, wheat, barley, rye, rice, millet, 
durra, and other small grains have been raised from time 
immemorial. America was the home of maize, or Indian 
corn, in many respects the most valuable of all grains, 
especially for the use of primitive peoples. By its culture 
the people of Mexico and Peru had laid the foundation for 
the nearest approach to civilization ever attained on this 
continent before its discovery by Europeans. 

The highest civilization is attainable only by a combina- 
tion of agriculture and stock raising. When to these in- 
dustries is added a knowledge of the art of smelting and 
refining iron ore, the physical basis of a high civilization 
is completed, and the steel age, in which all the leading 
peoples of the world are now living, is begun. 

Varieties of the Human Species. — Of all species of ani- 
mals, man is the most widely distributed. His intelli- 
gence enables him to live in all lands and all climates, 

DR. PHYS. GEOG. — 23 



386 



LIFE 



from Greenland to Tierra del Fuego, and from marshes 
and islands near sea level to the high Andes and Hima- 
layas. From his cradle land, which was probably some 
part of Eurasia, he seems to have migrated in all direc- 
tions, without definite purpose or destination, wherever the 
land connections of those remote times furnished a road. 
Led on by the pursuit of food, or driven from place to place 
by enemies, he penetrated every unoccupied land, and 




Fig. 330. 



while still in a very rude stage of culture took possession 
of most of the habitable world. In the struggle for ex- 
istence under such a great variety of conditions, men, 
like other animals, necessarily developed differently and 
unequally. Hence arose four distinct varieties or races, 
differing in physical and mental characteristics. The hot, 
moist, equatorial forests of central Africa produced a 
black race (Ethiopian), which spread over the whole of 
tropical Africa and the similar regions of the East Indies, 
which, together with Australia, formed a new center, where 
men of a somewhat different and lower type were devel- 



THE GEOGRAPHY OF MAN 387 

oped. The high, arid plateaus of central Asia produced 
a yellow race (Mongolian), which spread over nearly the 
whole of Asia and the neighboring islands. The American 
-continent produced a red race (American), which occupied 
its whole area for many centuries without interference 
from the rest of the world. North Africa and western 
Europe gave birth to a white race (Caucasian), which, in 
historic times, has spread thence over northern and south- 
ern Asia, America, south Africa, and Australia, dispossess- 
ing or gaining control of the aboriginal races. Figure 330 
shows the distribution of the four races as it was previous 
to the modern migrations which began in the sixteenth 
century. The table on p. 389 shows their distinctive char- 
acteristics and present distribution. 

Types of the Caucasian Race. — The peoples of each race, though 
alike in the characteristics mentioned on p. 389, differ in many minor 
details. Among the peoples of the Caucasian race there are three 
well-marked types. 

(1) North European or Teutonic. — Blond or florid, with flaxen or 
reddish, glossy hair, blue eyes, long skull, and tall stature. Scandina- 
vians, North Germans, Dutch, English, Scotch, Irish, and their descend- 
ants in America, Australia, and south Africa ; West Persians, Afghans, 
many Hindus, and some other peoples of southwest Asia. 

(2) Alpine. — Light brown or swarthy, with brown, wavy, dull hair, 
brown, gray, or black eyes, broad skull, and medium stature. Most 
French and Welsh, South Germans, Swiss, Russians, Poles, Bohe- 
mians, and other peoples of southeastern Europe ; Armenians, East 
Persians, and the peoples of the eastern Pacific islands. 

(3) Mediterranean. — Olive brown to almost black, with dark or black 
wiry hair, dark or black eyes, round skull, and small stature. Spanish 
and Portuguese and their descendants in America ; some French, Welsh, 
and Irish ; Italians and Greeks ; Berbers, Egyptians, and other peoples 
of north Africa; Arabs, Syrians, and other peoples of southwest Asia; 
some Hindus. 

The Population of the World according to races is esti- 
mated to be as follows : — 



3 88 



LIFE 





jC**f' : w* 




M 



Fig- 331.— An Ethiopian. 



Fig- 332- - A Mongolian. 





Fig- 333. —An American. 



Fig. 334. —A Caucasian. 



Caucasians . 
Mongolians . 
Ethiopians . 
Americans 
Total 



770,000,000 

540,000,000 

175,000,000 

22,000,000 

1,507,000,000 



THE GEOGRAPHY OF MAN 



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390 LIFE 

Civilized Men. — No people of Ethiopian race has ever 
risen, without help from some other race, above a condition 
of barbarism. The same is true of the American race 
with the exception of the Aztecs of Mexico and the Incas 
of Peru. Native civilization belongs chiefly to the Cauca- 
sian race, and in a lesser degree to the Mongolian. As 
in the case of other animals, man has attained his highest 
development in the north temperate zone, within which all 
the great civilizations of ancient and modern times have 
sprung up. The great centers of civilization have been 
located upon lowlands which either were traversed by 
large rivers, or were easily accessible from the sea, or both. 
This is true of China, India, Babylonia, Egypt, Greece, 
Italy, Great Britain, and the countries of western Europe. 

Man can live in the north frigid zone, but it furnishes only the bare 
necessities of life, and the whole of human energy must be expended 
in obtaining them. In the torrid zone the climate is oppressive and 
hardly permits prolonged exertion. Clothing and shelter are scarcely 
needed, and food can be procured without much effort or forethought. 
The luxuriance of plant and animal life is so great that man is over- 
whelmed by it and remains insignificant. In temperate climates food 
and clothing may be obtained in abundance, but only by the exercise 
of industry and invention. The inclement and unproductive winter 
makes it necessary to provide beforehand substantial shelter and a sup- 
ply of food. These conditions stimulate men to exert their physical 
and mental powers, and their efforts are rewarded with comforts and 
luxuries. Well-watered lowlands are very productive, especially of the 
cereal grains. The presence of navigable rivers or the sea renders 
travel and transportation easy, leading to commerce and that inter- 
change of ideas characteristic of enlightened peoples. 

Influence of Physical Features. — The degree of civiliza- 
tion, power, and influence attained by a state or people 
depends upon numerous physical factors belonging to the 
territory which it occupies. The latitude, distance from 
the sea, and relief of a country largely determine its 



THE GEOGRAPHY OF MAN 391 

temperature, rainfall, and soil, consequently its products 
and the occupations of its people. Switzerland, the Scotch 
highlands, and Norway produce a different type of people 
from those which inhabit the low plains to the south of 
them. A long and irregular coast line, with arms of the 
sea extending far into the land, constitutes one of the most 
favorable conditions for human occupation. The contrast 
between Europe and Africa, and between Great Britain 
and Australia, is in this respect very great. Mountain 
ranges act as barriers to rainfall, making it unequal upon 
opposite slopes, form the natural boundaries of states, and 
in their disturbed and dislocated structure expose veins of 
coal, iron, and other minerals. A network of rivers and 
lakes affords opportunities for internal commerce and fur- 
nishes water power for manufactures. The size or area of 
a country exerts no small influence upon the condition of 
its people. The area of the United States is so large that 
it includes all temperatures from sub-tropical to frigid, all 
rain belts from the heavy rainfall of the Pacific and Gulf 
coasts to the almost rainless deserts of the Great Basin, 
all relief forms from the low plains of the Atlantic coast 
and Mississippi basin to the high plateaus and mountains 
of the western half. Hence its agricultural and mineral 
products are so varied and abundant as to render it largely 
independent of the rest of the world, and to constitute the 
largest resources of natural wealth belonging to any one 
people in the world. 

Natural Resources. — ■ Civilized men are learning more 
and more how to modify the conditions of their environ- 
ment and to turn them to their own advantage. They are 
everywhere engaged in developing the natural resources 
of their country. These consist of three classes : (1) agri- 
cultural products, both vegetable and animal, which depend 



392 LIFE 

upon the soil, but also upon the energy supplied by the sun 
in the form of heat and light, and upon water vapor in the 
air ; (2) mineral products, such as coal, iron, copper, lead, tin, 
zinc, gold, silver, salt, and building stones, which are con- 
tained in the crust of the earth, and the quantity of which can 
not be increased ; (3) resources zvhich furnish pozver, such as 
coal, petroleum, natural gas, water power, and wind power, 
used to run machinery. Of all these, agricultural resources 
furnish the foundation of the state. While the quantity is 
not unlimited, it can be increased by skillful management 
so as to support a population more dense than now exists 
anywhere in the world except in China and India. Of min- 
eral resources, coal and iron ore — the indispensable mate- 
rials of modern civilization — are by far the most important. 
These are not now being formed, and the exhaustion of the 
supply seems to be a certainty of the remote future. The 
more extensive use of water power, as at Niagara Falls, will 
make the consumption of coal less rapid. The increasing 
use of machinery involves the use of greater quantities of 
iron and other metals and of fuel, the growth of manufac- 
ture, and the extension of commerce. The building of rail- 
ways has rendered most other means of land transportation 
useless or of less importance. The use of large and swift 
steamships has changed the sea from an impassable barrier 
to a means of easy communication between peoples. Within 
a century the world has shrunk for purposes of human 
intercourse to practically one tenth its former size. The 
most enterprising nations are extending their lines of com- 
merce and influence in every direction, and the more pro- 
lific peoples, like the British, Germans, Russians, and 
the people of the United States, are expanding by an- 
nexation and colonization to occupy, control, and develop 
nearly every portion of the habitable globe. 






APPENDIX I 
THE EQUIPMENT OF A GEOGRAPHICAL LABORATORY 1 

Geography can not be learned without suitable material and appli- 
ances any more profitably than physics or chemistry. The apparatus 
required consists of models, maps, pictures, specimens, and instruments 
for work in meteorology. 

Models. — Many models of especially instructive regions which have 
been adequately surveyed, are now available, but really good models are 
necessarily rather expensive. Those of crude and inaccurate construc- 
tion, and with vertical heights greatly exaggerated, are liable to teach 
more error than truth, and are worse than useless. The following mod- 
els (sometimes called relief-maps) are among the best and most useful. 




Fig 335. —Howell's model of the United States. 

The United States, Gulf of Mexico, and portions of the Atlantic and 
Pacific oceans, constructed as a section of a sphere 16 feet in diameter. 
Horizontal scale, 40 miles to 1 inch : vertical scale, 8 miles to 1 inch. 
Size 4 ft. 2 in. x8 ft. $125.00. 

1 See Journal of School Geography, 2, 170. 
393 



394 APPENDIX I 

A copy of the above on a scale of 120 miles to 1 inch. Size 1 ft. 6 in. 
x 2 ft. 10 in. Easily portable. $25.00. 

The Uinta and Wasatch mountains, showing folded and faulted 
mountains, canyons of Green River, escarpments, and dip slopes. 
Scale 4 miles to 1 inch. Vertical heights exaggerated 2 to 1. Size 
4 ft. X4 ft. 2 in. $125.00. 

The Grand Canyon of the Colorado and the plateaus of southern 
Utah. Horizontal and vertical scales 2 miles to 1 inch. Size 6 ft. 
x6ft. $125.00. 

The Henry Mountains and vicinity. Scale 2 miles to 1 inch. Size 
3 ft. x 3 ft. 6 in. $30.00. 

Stereogram of the Henry Mountains showing the same region as 
the preceding as it would be if the eroded material were restored. 
$12.00. 

Southern New England. Scale 2 miles to 1 inch. Size 5 ft. 7 in. 
x8 ft. 4 in. $135.00. 

The Chattanooga district. Scale 1 mile to 1 inch. Size 3 ft. 4 in. 
x 3 ft. 10 in. $50.00. 

New York. Horizontal scale 12 miles to 1 inch; vertical heights 
exaggerated 5 to 1. Size 2 ft. 1 in. x 2 ft. 10 in. $25.00. 

Mt. Shasta. Size 3 ft. 3 in. x 3 ft. 4 in. $40.00. 

Mt. Vesuvius. Size 2 ft. x 2 ft. 6 in. $10.00. 

These models are made and sold by Edwin E. Howell, 612, 17th St. 
N.W., Washington, D.C. 

Mr. Howell also furnishes a set of five models of the continents at 
$150.00. 

Professor John F. Newsom, Leland Stanford Junior University, Cali- 
fornia, furnishes the following six models : 

Morrisons Cove, Penn., showing anticlinal and synclinal folds. Size 
1.9 ft. x 2.3 ft. $35.00. 

Allamakee County, Iowa, showing topographic forms in a region of 
horizontal strata. Size 2.3 ft. x 2.5 ft. $20.00. 

Marysville Buttes, Cal., showing volcanic cone surrounded by sedi- 
mentary strata. Size 1.8 ft. x 1.8 ft. $12.00. 

Ideal Restoration of- Marysville Buttes, showing maximum develop- 
ment of a volcano. $5.00. 
■ Crater Lake, Oregon. Size 1.1 ft. x 1.4 ft. $7.00. 

Sectioned model of the Leadville region, Col., showing intense fold- 
ing, faulting, and igneous intrusions. Size 2.6 ft. x 3.2 ft. $85.00. 



EQUIPMENT OF A GEOGRAPHICAL LABORATORY 395 

Harvard Geographical Models, a set of three, each 25 x 19 inches, 
showing (1) Mountains Bordering the Sea, (2) Coastal Plain and 
Mountains, (3) Embayed Mountains. Ginn & Co., Boston. Per set, 

,$20.00. 

Jones's New Model of the Earth, mounted as a globe, 20 inches in 
diameter. Vertical scale exaggerated 20 times. A. H. Andrews & Co., 
Chicago. $50.00. 

Maps. — Large-scale maps for the wall or table are indispensable for 
the class room, and can now be obtained at small expense. 1 

Map of the Alluvial Valley of the Mississippi River. Scale 5 miles 
to 1 inch. 8 sheets. Per set, $1.00. Map of the Alluvial Valley of 
the Upper Mississippi River. 4 sheets. Per set, 70 cents. Map of 
the Lower Mississippi River in 32 sheets. Scale 1 mile to 1 inch. Per 
set, $1.60. Map of the Upper Mississippi River in 30 sheets. Per 
set, $1.50. Address Secretary Mississippi River Commission, St. 
Louis, Mo. 

Map of the Missouri River from its mouth to Three Forks, Mont. 
Scale 1 mile to 1 inch. 96 sheets, 5 cents per sheet. Address Secre- 
tary Missouri River Commission, St. Louis, Mo. 

Survey of the Northern and Northwestern Lakes. Price list may be 
obtained from United States Engineer's Office, Detroit, Mich. The 
Niagara Falls and Lake St. Clair charts are of especial value. 

United States Coast and Geodetic Survey Charts. An illustrated 
catalogue of charts may be obtained on request from the Superintendent, 
Washington, D.C. Old charts which have been superseded, but are 
not less valuable for teaching purposes, may often be obtained free. 

Topographical Atlas of New Jersey ; 20 sheets at 25 cents each, or 
the set, $5.00. Geological Survey of New Jersey, Trenton. 

Topographical Atlas of Massachusetts ; 54 sheets at 5 cents each, or 
the set $4.25. Topographical Survey of Massachusetts, Boston. 

Topographical Map of Rhode Island, $2.00. Topographical Survey 
of Rhode Island, Providence. 

Topographic Atlas of the United States. Published in sheets, many 
of which are accompanied by geological maps, pictures, and descriptive 
text, the collection being called a Folio (in the following pages folios 
are marked thus: *). Relief shown by contour lines. Single sheets 

1 Consult Governmental Maps for Use in Schools, Henry Holt & Co., N.Y. 
30 cents. Also Journal of School Geography, 1, 200. 



396 APPENDIX I 

5 cents, or $2 00 per 100. Folios, 25 cents each. Price list sent on appli- 
cation to the Director, U. S. G. S., Washington, D.C. Out of several 
thousand the following are especially useful : — 

Physiographic Types, Folio 1 : Ten maps with descriptive text : A 
Region in Youth, A Region in Maturity, A Region in Old Age, A 
Rejuvenated Region, A Young Volcanic Mountain, Moraines, Drum- 
lins, River Flood Plains, A Fiord Coast, A Barrier Beach Coast. 

Folio 2 : A Coast Swamp, A Graded River, An Overloaded Stream, 
Appalachian Ridges, Ozark Ridges, Ozark Plateau, Hogbacks, Volcanic 
Peaks, Plateaus and Necks, Alluvial Cones, A Crater. 

A single sheet map of the United States. Relief shown by nine shades 
of brown color; also with relief shown by contours. 

Marine Plains : Glassboro, N.J. 

Fluviatile Plains : Marysville,* Cal. 

Lacustrine Plains: Sierraville, Lassen Peak,* Cal.; Tooele Valley, 
Utah ; Disaster, Paradise, Nev. 

Glacial Plains : Marion, la. 

Dissected Plains : Spottsylvania, Farmville, Palmyra, Va. ; McCor- 
mick, Ga. ; Clanton, Ala. 

Upland Plains : Springfield, Bolivar, Tuscumbia, Fulton, Mo. ; Iola, 
Kan. 

Plateaiis : Fort Defiance, Ariz. ; Las Animas, Kit Carson, Lamar, 
Granada, Col. 

Dissected Plateaus : Mesa de Maya, Col. ; Marsh Pass, Ariz. ; Cold- 
water, Meade, Kan. ; Hazard, Salyersville, Warfield, Ky. ; Kanawha 
Falls, Nicholas, Huntersville, Hinton, W. Va. ; Scottsboro, Ala. ; Se- 
wanee, Tenn. ; Marshall, Ark. ; Gaines, Pa. 

Trenched Plateaus, Cliffs, Suites, Canyons : Kaibab, Echo Cliffs. 
Ariz. ; Escalante, Price River, Kanab, Utah. 

Denuded Plateaus, Escarpments, Outliers, Mesas : Watrous, Corazon, 
N. Mex. ; Sewanee,* Tenn. ; Kaaterskill, N.Y. ; East Tavaputs, Utah ; 
Tusayan, Ft. Defiance, Ariz. ; Abilene, Brownwood, Tex. 

Basin Ranges : Tooele Valley, Utah ; Disaster, Nev. ; Alturas, Cal. 

Rocky Mountains : Canyon City, Huerfano Park, Pikes Peak,* Platte 
Canyon, Telluride,* Col. ; Livingston,* Mont. ; Yellowstone National 
Park,* Wyo. 

Wasatch and Uinta Mountains : Salt Lake, Uinta, Utah. 

Black Hills: Rapid, S.D. 

* A folio (see p. 395). 






EQUIPMENT OF A GEOGRAPHICAL LABORATORY 397 

Appalachian Mountains: Lykens, Pottsville, Harrisburg, Hummels- 
town, Pa.; Monterey,* Franklin,* Estillville,* Va. ; Piedmont,* W.Va. ; 
Mt.Mitchell, Asheville, Pisgah, N.C. ; Briceville,* Cleveland,* Loudon,* 
Pikeville,* Kingston,* Chattanooga,* Tenn. ; Ringgold,* Atlanta, Ga. ; 
Gadsden,* Stevenson,* Ala. 

Mountain Highlands : Hawley, Chesterfield, Granville, Becket, Mass. ; 
Winsted, Bridgeport, Cornwall, Derby, Conn. ; Hackettstown, N.J. 

Volcanoes: Shasta, Marysville,* Cal. ; San Francisco Mt., Ariz.; 
Mt. Taylor, N. Mex. 

Lava Plains: Modoc Lava Bed, Cal.; Bisuka, Boise, Silver City, 
Nampa, Ida. 

Laccolitcs: San Rafael, Henry Mountains, Utah. 

Volcanic Dikes, Mesas, and Plugs: Absaroka,* Wyo. ; Elmoro,* 
Col. ; Greenfield, Holyoke,* Mass. 

Flood Plains: Donaldsonville, Mt. Airy, Pointe a la Hache, Gibson, 
Houma, La. ; Fort Payne, Ala. ; St. Louis (east sheet), Independence, 
Marshall, Mo. ; Junction City, Kan. ; Minden, Neb. 

Meandering Valleys : Versailles, Tuscumbia, Mo. ; Palo Pinto, Gran- 
bury, Tex. 

Transverse Valleys and Water Gaps: West Point, Tarrytown, Har- 
lem, N.Y. ; Harpers Ferry,* Va. ; Harrisburg, Delaware Water Gap, Pa. 

Filled Valleys : Lake, Wyo. ; Independence, Marshall, Mo. ; Disaster, 
Granite Ridge, Long Valley, Nev. 

Migrating Divides and Trellised Drainage : Doylestown, Pa. ; 
Dahlonega, Gainesville, Walhalla, Ga. ; Franklin, Pocahontas, Va. 

Hudson River : Hoosick, Troy, Albany, Coxsackie, Catskill, Pough- 
keepsie, N.Y. 

Cataracts and Gorges : Rochester (special), Niagara Falls (special), 
N.Y. ; Minneapolis, Minn. ; Great Falls, Mont. ; Yellowstone National 
Park,* Wyo. 

Moraines, Drumlins : Madison, Sun Prairie, Waterloo, Watertown, 
Oconomowoc, Wis. ; Charlestown, R.I. ; Stonington, Conn. 

Cirques: Anthracite and Crested Butte,* Col. 

Glacial Lakes : Webster, Mass. ; Madison, Geneva, Wis. 

Finger Lakes : Ithaca, Elmira, N.Y. 

Volcanic Lakes : Ashland, Crater Lake (special), Ore. 

Old Lake Outlets : Ottawa, Marseilles, Lasalle, Calumet, Des Plaines, 
111. ; Oneida, Oriskany, Schenectady, Cohoes, N.Y. 

* A folio (see p. 395). 



398 APPENDIX II 

River Terraces : Springfield, Mass. ; Hartford, Conn. 

Drowned Valleys and Fiords : Wicomico, Md. ; Fredericksburg,* 
Nomini,* Mt. Vernon, Va. ; Norwich, New London, Conn. ; Portland, 
Casco Bay, Boothbay, Me. 

Bays and Bars : Duxbury, Nahant, Boston, Mass. ; Ontario Beach, 
N.Y. ; Duluth, Minn. ; San Francisco, Cal. ; Seattle,* Tacoma,* 
Wash. 

Barrier Beaches and Spits : Sandy Hook, Asbury Park, Barnegat, 
Long Beach, N.J. ; Marthas Vineyard, Gay Head, Provincetown, Mass. 

Foreign Maps. — Most of the European countries have published 
governmental maps on a large scale, many of which are models of the 
cartographic art. Consult " Large-Scale Maps as Geographical Illustra- 
tions," by Davis, Journal of Geology, 4, 484. 

Pictures and Lantern Slides. — Pictures are now so common and 
cheap that a very good collection can be made from magazines, railroad 
advertisements, and newspapers. A few collections of large photo- 
graphic or autotype pictures containing many illustrations of geographical 
features have been made. Scenes from Every Land, 500 photographs, 
J. W. Jones, Springfield, Ohio, $5.00; America Photographed, 210 
views, Donahue & Hennebery, Chicago, $1.00; and Our Own Country, 
500 pictures with descriptive text, The National Co., St. Louis, Mo., 
$3.50, may be recommended. 

The lantern for projection has become the common adjunct of 
school instruction. It is now supplied by all dealers in scientific instru- 
ments. E. E. Howell, Washington, D.C., supplies a list of slides 
selected by Professor Davis. The American Bureau of Geography, 
Winona, Minn., has undertaken to supply good photographs and slides. 
Announcements are made in its Bulletin, quarterly, $1.00 per year. 

Suggestive exercises in laboratory work in geography will be found in 
Journal of School Geography, 1, 172, 204; 3, 368. 



APPENDIX II 

METEOROLOGICAL INSTRUMENTS 1 

The Measurement of Temperature. — Standard Thermometer ($2.75). 
Temperature is measured by a thermometer. This instrument consists 
of a small glass tube with a bulb at one end. The bulb and part of the 

* A folio (see p. 395). 1 See Journal of School Geography, 3, 241. 



METEOROLOGICAL INSTRUMENTS 



399 



tube are filled with mercury or alcohol, the air is removed 
and the tube closed. The bulb is then placed in melting 
ice and the point at which the top of the column stands 
is marked 32 and called the freezing point. The bulb is 
then placed in the steam above boiling water, and the 
point at which the top of the column stands is marked 
212 and called the boiling point. The space between is 
divided into 180 equal degrees and the graduation is ex- 
tended below 32 on the same scale. This is called the 
Fahrenheit scale. The Centigrade scale, which marks 
the freezing point o° and the boiling point ioo°, is also 
much used. In determining the error of a common ther- 
mometer by comparison with a standard, the comparison 
should be made at as many different points in the scale as 
possible. 

Maximum and Minimum Thermometers ($8.25). These 
instruments should be mounted together upon a board as 
shown in Fig. 337. The tube of the maximum is bent and 
constricted just above the bulb. As the temperature rises, 
the mercury passes up the tube. When the temperature 
falls, the column breaks at the constriction and remains at 
the highest point reached. After reading, the instrument should be set 
by rotating it rapidly around the pin at its upper end. 

The minimum is filled with alcohol and contains a steel index. 
When the temperature falls, the index is dragged downward by the 
surface tension of the alcohol. When the temperature rises, the index 



Fig. 336. 

Standard 

thermometer. 




Fig. 337- 



is left behind at the lowest point reached. The instrument is set 
by raising the bulb until the index slides down to the surface of the 
alcohol. 

Shelter. Thermometers should be exposed in a latticed shelter in an 
open space away from buildings and four to ten feet above the ground. 



400 



APPENDIX II 



If a shelter is not available, they may be placed outside 
a north window in such a position that they may be 

read without open- 
ing the window. 

The Measurement 
of Pressure. — The 
Mercurial Barom- 
eter ($5.75 to $30.- 
00) consists of a 
glass tube and cup 
containing mer- 
cury, inclosed in 
a metal tube for 
protection and pro- 
vided with devices 
for convenient and 
accurate reading. 
The distance to be 
measured is the 




Fig- 338. —A latticed shelter. 



difference between the level of the mercury in the tube 
and its level in the cup. As the mercury falls in the 
tube it rises in the cup, and vice versa : therefore it is 
necessary to bring the mercury in the cup to a certain 
fixed level before reading. The cup has a leather bot- 
tom which is pressed by the screw C, Fig. 339. By 
turning this screw, the level of the mercury is adjusted 
so that its surface just touches the point of an ivory pin 
at B. This is the zero point of the scale. The zero 
is sometimes marked by a black line on the outside of 
the cup. In the upper part of the metal tube two slots 
are cut so that the mercury column can be seen. Two 
metal pieces slide in the slots and are moved by the 
screw E. Placing the eye in a position where the lower 
edge of the front piece just hides the lower edge of the 
back piece, move both pieces until their lower edges 
just cut off the light between themselves and the sur- 
face of the mercury at the center of the tube. To the 
metal tube is fastened a scale graduated into inches and 
tenths. The hundredths are read from the vernier, or 




Fig. 339. — Mercu- 
rial barometer. 



METEOROLOGICAL INSTRUMENTS 



401 



@ 








sliding piece, being indicated by the line on the ver- 
nier which coincides with a line on the fixed scale. 
The reading in Fig. 340 is 29.25 inches. Some 
barometers are read without a vernier. 

The mercury of the barometer is expanded by 
heat, and when the temperature is high it requires 
a longer column to balance the air pressure than 
when the temperature is low. It is therefore neces- 
sary to read the attached thermometer at D, and to 
correct the barometer reading for temperature. 

In drawing isobaric maps the observed pressures 
are generally reduced to what they would be if the 
observing station were at sea level. This is done 
by adding the length of a column of mercury which 
would balance a column of air extending from sea 
level up to the station. Tables for the reduction of 
the barometer reading to 32 F. and to sea level are 
given on pp. 406, 407, and in Ward's Practical Exer- 
cises in Elementary Meteorology. 

The Aneroid Barometer ($6.00 to $15.00) indi- 
cates pressure by the expansion and contraction of 
a vacuous metal box, the movement of which is communicated to a 
pointer like a clock hand, which revolves over a circular scale. If com- 
pensated for temperature, this instrument 
is accurate and convenient. 

The Measurement of Humidity. — The 
Hygrometer ($6.50) consists of two simi- 
lar thermometers mounted upon a board. 
The bulb of one is kept wet by being cov- 
ered with a lamp wick which dips into a 
cup of pure water. The evaporation of the 
water cools the mercury and makes it stand 
lower than in the dry thermometer. If the 
air is dry, evaporation is rapid, and the dif- 
ference between the two thermometers 
may be 15 or 20 . If the air is damp, 
evaporation is slow and the difference is 
small. The indications of the hygrometer are made definite by refer- 
ence to tables. See pp. 408, 409; or Psychrometric Tables, published 



Fig- 340/ 




Fig. 34 1 -— Aneroid barometer. 



DR. PHYS. C.EOG. 



24 



402 



APPENDIX II 



by the U.S. Weather Bureau (price 10 cents) ; or Ward's Practical 
Exercises in Meteorology. The wet bulb should be fanned before 

reading, to prevent the accumulation of 
vapor near the instrument. 

Any thermometer may be made to serve 
as a hygrometer by covering its bulb with 
wet muslin and swinging it around in the 
air by an attached cord until the mercury 
ceases to fall. Its reading should be com- 
pared with that of a similar dry thermom- 
eter. This instrument is called the sling 
ftsydirometer. 

The Measurement of Precipitation. — The 
Rain Gauge ($1.25 to $5.25) is a metal 
cylinder having an inside diameter of 8 
inches. The receiver is funnel-shaped and 
carries the water into a measuring tube 
whose area of cross section is one tenth 
that of the receiver. Thus one tenth of an 
inch of rainfall gives a depth of 1 inch of 
water in the tube. The depth is measured 
by a stick graduated in inches and tenths. 
The gauge should be mounted in .a verti- 
cal position several feet above the ground 
in an open space at a distance from build- 




Fig. 342. — Hygrometer. 

ings and trees, and read 
emptied every morning. Snow 
is estimated as water after melt- 
ing. 

The Tliermograph and Baro- 
graph ($30.00 each) are instru- 
ments which make continuous 
records of temperature and pres- 
sure upon a strip of paper. 
They are indispensable for a 
thorough and comprehensive 
study of the weather. Instruc- 
tions for their use are furnished 
by the dealers. 



lii-iil Section. 




Scale 
Fig. 343. -Rain gauge. 



METEOROLOGICAL INSTRUMENTS 



403 




Fig. 344. — Thermograph. 

Measurement of the Wind. — The direction of the wind can not be 
determined with accuracy without the use of a vane. The best vane is 




Fig- 345— Barograph, 
made of wood 6 feet long, and has a divided tail, the two parts making 
an angle of 22|°. It should be placed in a position above all trees and 

buildings. 

& Vertical Sec. 




Fig. 346. — Vane. 



4°4 



APPENDIX II 



The Anemometer is a windmill with cup-shaped arms which records 
by the number of its revolutions the wind velocity. For purposes of 

elementary study it is suffi- 
cient to estimate the wind 
velocity according to the fol- 
lowing scale, 
o. Calm. 

i . Light, 2-5 miles per hour, 
moving leaves. 

2. Moderate, 7-10 miles, 
moving branches. 

3. Brisk, 1 8-20 miles, swaying 
branches, blowing up dust. 

4. High, 27-30 miles, sway- 
ing trees, blowing up twigs. 

5. Gale, 45-50 miles, break- 
ing branches, loosening 
bricks, signs, etc. 

6. Hurricane, 75 miles, de- 
fig- 347- — Anemometer. straying everything. 

Meteorological instruments are furnished by many firms : Queen & Co., 
Philadelphia, Pa.; H. J. Green, 1191 Bedford Ave., Brooklyn, N.Y. ; 
L. E. Knott Apparatus Co., Boston, Mass.; Julien P. Friez, Balti- 
more, Md. 

Form for Meteorological Record 






3 

O 


3 

ft 


Temperature. 


i 

ft 


Wind. 


Cloud. 


Precipit. 


Date. 


>> 

Q 








c 
g 

u 
Q 


>> 

V 

> 


3 


c 

S 
< 


-a 
c 

2 


c 


E 
< 











The signs used on the U. S. Weather Maps may be used for wind 
direction, amount of cloud, and kind of precipitation. 

A laboratory course in elementary meteorology is outlined in the 
Journal of School Geography, 1, 41 ; 2, 2, 56, 96, 104, 139. 



METEOROLOGICAL INSTRUMENTS 405 

Use of the Tables. — The tables on pp. 406-409 are almost self- 
explanatory ; but students not accustomed to the use of such tables 
should study the following explanations of their use. 

(1) Temperature corrections for barometer readings (p. 406), some- 
times called "tables for reducing barometer readings to 32 ." In the 
first column, in heavy-faced type, are temperatures from o° to ioo°, and 
on the same line with each temperature are printed, in ordinary light- 
faced type, eight different numbers in as many columns. The correc- 
tion to be applied is the number in the column with the heading (in 
heavy-faced type) which is nearest the barometer reading. For instance, 
if a barometer reads 28.21, and the attached thermometer 70 , the tem- 
perature correction is found in the column headed 28, and on the line 
with 70 . Applying the correction, .11, we have 28.10 as the corrected 
reading. For temperatures below 28 the correction is to be added, but 
for temperattires above 28 the correction is to be subtracted. 

(2) Table for reducing barometer readings to sea level (p. 407). As 
in the other tables, the known data are printed in heavy-faced type, and 
the quantities given by the table in light-faced type. For any particular 
elevation, the amount to be added varies with the temperature. For 
instance, if the elevation is 1200 feet, the amount is 1.43 at o° tempera- 
ture, 1.40 at io°, and so on. If the barometer reading at elevation 1200 
feet, corrected for temperature, is 28.10, and the air temperature is 70 , 
the amount to be added is found from the table to be 1.24. The read- 
ing as reduced to sea level is therefore 29.34. 

(3) Table for finding relative humidity (pp. 408, 409). The various 
columns in the three parts of this table are headed by numbers in heavy- 
faced type from 1 to 42, each representing a possible difference between 
the reading of the dry thermometer and that of the wet-bulb ther- 
mometer at the same time, the reading of the wet-bulb thermometer being 
always the lower. On each line of the table are printed, in light-faced 
type, the various percentages of relative humidity for a certain tempera- 
ture (printed in heavy-faced type at the left of the table) as given by 
the dry thermometer. For instance, if the temperature (dry thermom- 
eter) is 70 , a difference of i° in the readings of the dry and wet-bulb 
thermometers indicates a relative humidity of 95 per cent (p. 408) ; a 
difference of 2 , a relative humidity of 90 per cent ; a difference of 15 , 
a relative humidity of 37 per cent (p. 409) ; and so on. 

The table on pp. 408, 409, is correct for a barometrical pressure of 
29 inches, and approximately correct for all other ordinary pressures. 



406 



APPENDIX II 



Temperature Corrections for Barome- 
ter Readings: Amounts to be Added 






14" 
i6° 

i8° 



24" 
26 

28° 



Barometer reading, inches 
24 25 26 27 28 29 30 31 



06 


.07 


•07 


.07 


.07 


.08 


.08 


06 


.06 


.06 


.07 


.07 


•07 


•07 


0=; 


.06 


.06 


.06 


.06 


.07 


.07 


•05 


•05 


■o.S 


.06 


.06 


.06 


.06 


05 


•05 


•05 


•05 


•05 


•05 


.Ob 


04 


.04 


.04 


•OS 


•OS 


•OS 


•OS 


04 


.04 


.04 


.04 


.04 


.04 


•OS 


03 


•03 


.04 


.04 


.04 


•04 


.04 


°3 


•03 


•03 


•03 


•03 


•03 


•03 


02 


.02 


•03 


■03 


•03 


■03 


•03 


02 


.02 


.02 


.02 


.02" 


.02 


.02 


01 


.02 


.02 


.02 


.02 


.02 


.02 


01 


.01 


.01 


.01 


.01 


.01 


.01 


01 


.01 


.01 


.01 


.OI 


.01 


.01 


00 


.00 


.00 


.00 


.00 


.00 


.00 



Temperature Corrections for Barome- 
ter Readings: Amounts to be Subtracted 






29" 
30° 

3I o 

32 
33° 
34° 
35° 
3&° 
37° 
38 

39° 
40 

41° 
42 
43° 
44° 

45° 
46° 

47 
48 

49° 
50 

5i° 
52 

53° 



Barometer reading, inches 
24 25 26 27 28 29 30 31 



.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.00 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.01 


.02 


.01 


.01 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


.02 


•03 


•03 


.02 


.02 


.02 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


•03 


■03 


■03 


•03 


•03 


•03 


•03 


•03 


•°3 


•03 


•03 


.04 


.04 


•03 


•03 


■03 


•04 


.04 


.04 


.04 


•03 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


.04 


•OS 


.04 


.04 


.04 


.04 


.04 


•OS 


•OS 


.04 


.04 


.04 


•05 


•05 


•OS 


•OS 


.04 


.04 


•OS 


•OS 


•05 


•OS 


•o.S 


.04 


•OS 


•05 


•OS 


•05 


•°5 


.ob 


•OS 


•OS 


•OS 


•OS 


•OS 


.06 


.06 


•OS 


•OS 


.os 


.ob 


.ob 


.ob 


.ob 


.os 


•OS 


.06 


.06 


.ob 


.06 


.ob 


•OS 


.06 


.06 


.06 


.06 


.06 


.07 



.02 

•03 
■03 

■03 
.04 
.04 
.04 
•04 

•OS 
■05 
•OS 
•05 
.06 

.06 

.ob 

.07 I 

.07 1 



Temperature Corrections for Barome- 
ter Readings: Amounts to be Subtracted 



a. S 
SB 



54° 

55° 
5°° 

57 n 

58 

59° 
6o° 
6i° 
62 
63 
64° 
65 
66° 

67° 
68° 
69 
70 

7i° 
72- 

73° 
74° 

75 o 
76 o 

77° 
78° 

79° 
8o° 
8i° 
82° 

83° 
84° 

85° 
86° 

87° 



90 u 
9 i° 
92° 
93° 
94° 

95° 
96° 

97° 
98° 

99° 

100° 



Barometer reading, inches 
24 25 26 27 28 29 30 31 



.06 


.06 


.06 


.06 


.ob 


•07 


•07 


.06 


.06 


.06 


.06 


•07 


•07 


•07 


.ob 


.ob 


.ob 


•07 


.07 


.07 


•07 


.ob 


.ob 


.07 


•07 


•07 


.08 


.08 


.ob 


.07 


.07 


•07 


•°7 


.08 


.08 


.07 


.07 


.07 


.07 


.08 


.08 


.08 


.07 


.07 


•07 


.08 


.08 


.08 


.09 


•07 


.07 


.08 


.08 


.08 


•09 


.09 


.07 


.08 


.08 


.08 


.09 


.09 


.09 


.08 


.08 


.08 


.08 


.09 


.09 


.09 


08 


.08 


.08 


.09 


.09 


.09 


.10 


08 


.08 


•09 


.09 


.09 


.10 


.10 


08 


.09 


.09 


.09 


.10 


.10 


.10 


08 


.09 


.09 


.09 


.10 


.10 


.10 


.09 


•09 


.09 


.10 


.10 


.10 


.11 


•09 


.09 


.10 


.10 


.10 


.11 


.11 


■09 


.09 


.10 


.10 


.11 


.11 


.11 


.09 


.10 


.10 


.10 


.11 


.11 


.12 


.09 


.10 


.10 


.11 


.11 


.11 


.12 


.10 


.10 


.10 


.11 


.11 


.12 


.12 


.10 


.10 


.11 


.11 


.12 


.12 


.12 


.10 


.11 


.11 


.11 


.12 


.12 


•13 


.10 


.11 


.11 


.12 


.12 


.12 


• IS 


.11 


.11 


.11 


.12 


.12 


•13 


•13 


.11 


.11 


.12 


.12 


•1.3 


•1.3 


•1.3 


.11 


.11 


.12 


.12 


•13 


•13 


.14 


.11 


.12 


.12 


•13 


•13 


•14 


•14 


.11 


.12 


.12 


■ z 3 


•13 


.14 


■14 


.12 


.12 


■13 


•13 


.14 


•14 


•i.S 


.12 


.12 


•13 


•13 


.14 


•14 


•is 


.12 


•13 


•1-3 


.14 


•14 


•is 


•is 


12 


•13 


•13 


.14 


.14 


•i.S 


•i.S 


12 


•13 


■14 


•14 


•i.S 


•i.S 


.ib 


1.3 


■13 


•14 


•14 


•is 


•is 


.ib 


M 


•13 


■14 


■i.S 


•i.S 


.ib 


.ib 


13 


.14 


.14 


•IS 


•is 


.ib 


.ib 


13 


.14 


.14 


•IS 


.16 


.ib 


•17 


14 


.14 


•i.S 


•i.S 


.ib 


.lb 


■17 


14 


.14 


•IS 


■i.S 


.lb 


•17 


•17 


H 


•IS 


■IS 


.ib 


.lb 


•17 


•17 


14 


•15 


•15 


.ib 


■17 


•17 


.18 


14 


•is 


.16 


.16 


•17 


•17 


.18 


i.S 


•i.S 


.ib 


.ib 


•17 


.18 


.18 


IS 


•is 


.ib 


•17 


•17 


.18 


.19 


i.S 


.lb 


.ib 


■17 


.18 


.18 


•19 


is 


.ib 


•17 


•17 


.18 


.18 


•19 


is 


.16 


•17 


■17 


.18 


•19 


■19 



METEOROLOGICAL INSTRUMENTS 



407 



TABLE FOR REDUCING BAROMETER READINGS TO SEA LEVEL: 
Amounts to be added 



Elevation 










Temperature 










in feet 


0° 


10° 


20° 


30° 


40° 


50° 


6o° 


70 


8o° 


90 


IOO 


.12 


.12 


.12 


.12 


.11 


.11 


.11 


.11 


.10 


.10 


200 


.24 


.24 


•23 


■23 


.22 


.22 


.22 


.21 


.21 


.20 


300 


•36 


•36 


•35 


•34 


■34 


■33 


■32 


•32 


•31 


•30 


400 


.48 


•47 


.46 


.46 


■45 


■44 


43 


.42 


.41 


.40 


500 


.60 


•59 


•58 


•57 


•56 


•55 


•54 


•53 


•52 


•5i 


600 


.72 


•71 


.69 


.68 


.67 


•65 


.64 


■63 


.62 


.61 


700 


.84 


.82 


.81 


•79 


.78 


.76 


•75 


■73 


•72 


•71 


800 


.96 


•94 


•92 


.90 


.88 


.87 


•85 


.84 


.82 


.81 


900 


1.08 


1.06 


1.03 


1. 01 


•99 


•97 


.96 


•94 


.92 


.90 


1000 


1.20 


1. 17 


i-i5 


1. 12 


1. 10 


1.08 


1.06 


1.04 


1.02 


I. CO 


IIOO 


i-3i 


1.29 


1.26 


1.23 


1. 21 


1. 19 


1.16 


1. 14 


1. 12 


1. 10 


1200 


i-43 


1.40 


1-37 


1-34 


1.32 


1.29 


1.27 


1.24 


1.22 


1.20 


1300 


i-55 


i-5i 


1.48 


1-45 


1.42 


1.40 


i-37 


i-35 


1.32 


1.30 


1400 


I.b6 


1.63 


i-59 


1.56 


i-53 


1.50 


1.47 


1-45 


1.42 


1.40 


1500 


1.78 


1.74 


1.70 


1.67 


1.64 


1.61 


1.58 


i-55 


1-52 


1.49 


l600 


1.89 


i-8 S 


1.81 


1.78 


1.74 


1.71 


1.68 


i.6 S 


1.62 


i-59 


1700 


2.00 


1.96 


1.92 


1.89 


1.85 


1.81 


1.78 


i-75 


1.72 


1.69 


1800 


2.12 


2.07 


2.03 


1.99 


i-95 


1.92 


1.88 


1.85 


1.82 


1.78 


1900 


2.23 


2.19 


2.14 


2.10 ■ 


2.06 


2.02 


1.98 


i-95 


1.91 


1.88 


2000 


2-34 


2.30 


2.25 


2.21 


2.16 


2.12 


2.08 


2.05 


2.01 


1.97 


2100 


2.46 


2.41 


2.36 


2.31 


2.27 


2.22 


2.18 


2.14 


2.10 


2.07 


2200 


2-57 


2.52 


2-47 


2.42 


2-37 


2-33 


2.28 


2.24 


2 20 


2.16 


23OO 


2.68 


2.63 


, 2 -57 


2.52 


2.47 


2 43 


2.38 


2-34 


2.30 


2.26 


24OO 


2.79 


2-73 


2.68 


2.63 


2.58 


2-53 


2.48 


2.44 


2.40 


235 


2500 


2.90 


2.84 


2.79 


2-73 


2.68 


2.63 


2.58 


2-54 


2.49 


245 


2600 


3.01 


2-95 


2.89 


2.84 


2.78 


2-73 


2.68 


2.63 


2.58 


2-54 


270O 


3.12 


3.06 


3.00 


2.94 


2.88 


2.83 


2.78 


2-73 


2.68 


2.63 


2800 


3-23 


3.16 


3.10 


3-04 


2.98 


2-93 


2.88 


2.82 


2.77 


2-73 


29OO 


3-34 


3- 2 7 


3.21 


3-15 


3-«9 


3-o3 


2.97 


2.92 


2.87 


2.82 


3000 


3-45 


3-38 


3-3i 


3-25 


3-19 


3-13 


3-«7 


3.02 


2.96 


2.91 


3100 


3-5° 


3-49 


342 


3-35 


3-29 


3-23 


3- J 7 


3-n 


3.06 


3.00 


3200 


3.66 


3-59 


3-52 


345 


3-39 


3-32 


3.26 


3.21 


3-i5 


3.10 


33°o 


3-77 


3-69 


3.62 


3-55 


349 


342 


3-36 


3-3o 


3-24 


3-i9 


3400 


3.88 


3.80 


3-72 


3-65 


3-59 


3-52 


346 


34° 


3-34 


3.28 


35oo 


3-98 


3-9C 


3.82 


3-75 


3.68 


3.62 


3-55 


349 


343 


3-37 


3600 


4.09 


4.01 


3-93 


3-85 


3-78 


3-7i 


3-65 


3-58 


3-5 2 


• 346 


3700 


4.19 


4.1 1 


4-«3 


3-95 


3.88 


3.81 


3-74 


3-67 


3.61 


3-55 


3800 


4-3° 


4.21 


4-i3 


4-°5 


3-98 


3-9° 


3-83 


3-77 


370 


3- 6 4 


3900 


4.40 


4-32 


4-23 


4-15 


4.08 


4.00 


3-93 


3.86 


3-79 


3-73 


4000 


4-51 


4.42 


4-33 


4-25 


4.17 


4.10 


4.02 


3-95 


3-89 


3.83 


4100 


4.61 


4-S 2 


443 


4-35 


4.27 


4.19 


4.12 


4-«5 


3-98 


3-91 


4200 


4.71 


4.62 


4-53 


445 


4-37 


4.29 


4.21 


4.14 


4.07 


4.00 


4300 


4.82 


4.72 


4.63 


4-54 


4.46 


4-38 


4-3o 


4-23 


4-15 


4.08 


4400 


4.92 


4.82 


4-73 


4.64 


4-56 


447 


4-39 


4-3 2 


4.24 


4.17 


4500 


5.02 


4.92 


4.84 


4-74 


4-65 


4-57 


449 


4.41 


4-33 


4.26 


4600 


5-12 


S-02 


4-93 


4.84 


4-75 


4.66 


4-58 


4-5° 


4.42 


4-35 


4800 


5-32 


5.22 


5.12 


5.02 


4-93 


4-85 


4.76 


4.68 


4.60 


4-52 


5000 


5-52 


542 


5-32 


5- 2 2 


5.12 


5-°3 


4.94 


4.86 


4-77 


4.69 



4o8 



APPENDIX II 



TABLE FOR FINDING RELATIVE HUMIDITY: Percentages 



Dry 
therm. 






Difference between Dry and Wet-bulb Thermometers 






(air 
temp.) 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


12 


13 


r 4 





68 


35 


3 
























2 


7i 


41 


12 
























4 


73 


46 


19 
























6 


75 


50 


25 


1 






















8 


77 


54 


3i 


9 






















10 


79 


57 


36 


.15 






















12 


80 


60 


4i 


21 


3 




















14 


82 


63 


45 


27 


10 




















16 


83 


66 


■ 49 


33 


16 





















18 


84 


68 


53 


38 


22 


7 


















20 


85 


70 


56 


42 


28 


14 


















22 


86 


72 


59 


45 


32 


19 


7 
















24 


87 


74 


61 


49 


36 


24 


12 

















26 


88 


75 


64 


52 


40 


29 


18 


7 














28 


88 


77 


66 


55 


44 


33 


23 


12 


2 












30 


89 


78 


68 


57 


47 


37 


27 


17 


8 












32 


90 


79 


69 


60 


50 


41 


3i 


22 


13 


4 










34 


90 


81 


72 


62 


53 


44 


35 


27 


18 


9 


1 








36 


9i 


82 


73 


65 


56 


48 


39 


3i 


23 


14 


6 








38 


9i 


83 


75 


67 


59 


5i 


43 


35 


27 


19 


12 


4 






40 


92 


84 


76 


68 


61 


53 


46 


38 


3i 


23 


16 


9 


2 




42 


92 


85 


77 


70 


62 


55 


48 


41 


34 


28 


21 


14 


7 





44 


93 


85 


78 


71 


64 


57 


5i ■ 


44 


37 


3i 


24 


18 


12 


5 


46 


93 


86 


79 


72 


65 


59 


53 


46 


40 


34 


28 


22 


16 


10 


48 


93 


87 


80 


73 


67 


60 


54 


48 


42 


36 


3i 


25 


19 


14 


50 


93 


87 


81 


74 


68 


62 


56 


50 


44 


39 


33 


28 


22 


17 


52 


94 


88 


81 


75 


69 


63 - 


58 


52 


46 


41 


36 


30 


25 


20 


54 


94 


88 


82 


76 


70 


65 


59 


54 


48 


43 


38 


33 


28 


23 


56 


94 


88 


82 


77 


71 - 


66 


61 


55 


50 


45 


40 


35 


3i 


26 


58 


94 


89 


83 


77 


72 


67 


62 


57 


52 


47 


42 


38 


33 


28 


60 


94 


89 


84 


78 


73 


68 


63 


58 


53 


49 


44 


40 


35 


31 


62 


94 


89 


84 


79 


74 


69 


64 


60 


55 


50 


46 


41 


37 


33 


64 


95 


90 


85 


79 


75 


70 


66 


61 


56 


52 


48 


43 


39 


35 


66 


95 


90 


85 


80 


76 


7i 


66 


62 


58 


53 


49 


45 


41 


37 


68 


95 


90 


85 


81 


76 


72 


67 


63 


59 


55 


5i 


47 


43 


39 


70 


95 


90 


86 


81 


77 


72 


68 


64 


60 


56 


52 


48 


44 


40 


72 


95 


91 


86 


82 


78 


73 


69 


65 


61 


57 


53 


49 


46 


42 


74 


95 


9i 


86 


82 


78 


74 


70 


66 


62 


58 


54 


5i 


47 


44 


76 


96 


9i 


87 


83 


78 


74 


70 


67 


63 


59 


55 


52 


48 


45 


78 


96 


9i 


87 


83 


79 


75 


71 


67 


64 


60 


57 


53 


50 


46 


80 


96 


9i 


87 


83 


79 


76 


72 


68 


64 


61 


57 


54 


5i 


47 


84 


96 


92 


88 


84 


80 


77 


73 


70 


66 


63 


59 


56 


53 


SO 


88 


96 


92 


88 


85 


81 


78 


74 


71 


67 


64 


61 


58 


55 


52 


92 


96 


92 


89 


85 


82 


78 


75 


72 


69 


65 


62 


59 


57 


54 


96 


96 


93 


89 


86 


82 


79 


76 


73 


70 


67 


64 


61 


58 


55 


100 


96 


93 


90 


86 


83 


80 


77 


74 


7i 


68 


°S 


62 


59 


57 



METEOROLOGICAL INSTRUMENTS 



409 



TABLE FOR FINDING RELATIVE HUMIDITY : Percentages {Continued) 



Dry 
therm 






Difference between Dry and Wet-bulb Thermometers 






(air 

temp.] 


15 


16 


17 


18 


19 


20 21 22 


23 24 25 26 


27 


28 


46 


4 




























48 


8 


3 


























50 


12 


7 


2 
























52 


15 


10 


6 

























54 


18 


14 


9 


5 























50 


21 


17 


12 


8 


4 




















S« 


24 


20 


15 


11 


7 


3 


















60 


27 


22 


18 


14 


10 


6 


2 
















62 


29 


25 


21 


J 7 


13 


9 


b 


2 














04 


3i 


27 


23 


20 


ib 


12 


9 


5 


2 












66 


33 


29 


2b 


22 


18 


15 


11 


8 


5 


1 










68 


35 


3i 


28 


24 


21 


17 


14 


11 


8 


4 


1 








70 


37 


33 


30 


26 


23 


20 


17 


13 


10 


7 


4 


1 






72 


39 


35 


32 


23 


25 


22 


19 


16 


13 


10 


7 


4 


I 




74 


40 


37 


34 


30 


27 


24 


21 


18 


15 


12 


9 


7 


4 


I 


76 


42 


3« 


35 


32 


29 


2b 


23 


20 


17 


H 


12 


9 


b 


4 


78 


43 


40 


37 


34 


31 


28 


25 


22 


19 


ib 


14 


11 


9 


b 


80 


44 


4i 


38 


35 


32 


29 


27 


24 


21 


18 


16 


13 


11 


8 


82 


4 b 


43 


40 


•37 


34 


3i 


28 


25 


23 


20 


18 


15 


13 


10 


84 


47 


44 


4i 


3« 


35 


32 


30 


27 


25 


22 


20 


17 


15 


12 


86 


48 


45 


42 


39 


37 


34 


3i 


29 


2b 


24 


21 


19 


17 


14 


88 


49 


46 


43 


4i 


38 


35 


33 


30 


28 


25 


23 


21 


18 


ib 


90 


50 


47 


44 


42 


39 


37 


34 


32 


29 


27 


24 


22 


20 


18 


92 


5i 


48 


45 


43 


40 


38 


35 


33 


30 


28 


2b 


24 


22 


19 


94 


52 


49 


4 b 


44 


4i 


39 


3^ 


34 


32 


29 


27 


25 


23 


21 


90 


53 


50 


47 


45 


42 


40 


37 


35 


33 


3i 


29 


2b 


24 


22 


98 


53 


5i 


48 


4 b 


43 


41 


39 


3b 


34 


32 


30 


28 


2b 


24 


100 


54 


52 


49 


47 


44 


42 


40 


37 


35 


33 


3i 


29 


27 


25 



TABLE FOR FINDING RELATIVE HUMIDITY: Percentages (Continued) 



Dry 
therm. 






Difference between Dry and Wet-bulb Thermomete 


rs 






(air 
temp.) 


29 


30 


31 32 


33 34 35 3<5 37 3« 39 4° 4* 42 


76 


1 




























78 


4 


1 


























80 


b 


4 


1 
























82 


8 


6 


4 


1 






















84 


10 


8 


6 


4 


2 




















86 


12 


10 


8 


b 


4 


2 


















88 


14 


12 


10 


8 


6 


4 


2 
















90 


16 


14 


12 


10 


8 


6 


4 


2 















92 


17 


15 


13 


11 


9 


8 


b 


4 


2 













94 


19 


17 


15 


1.3 


11 


9 


8 


b 


4 


2 


1 








96 


20 


18 


17 


1.5 


13 


11 


9 


7 


6 


4 


3 


I 






98 


22 


20 


18 


ib 


14 


1.3 


11 


9 


7 


6 


4 


3 


I 




100 


23 


21 


19 


18 


ib 


14 


12 


11 


9 


7 


6 


4 


3 


1 



APPENDIX III 

THE CONSTRUCTION OF A WEATHER MAP 

The student will learn to read a weather map more rapidly and under- 
stand it more thoroughly by first making one. The table on p. 411 
gives the data sent into the United States Weather Bureau from all the 
stations on the morning of March 15, 1899. 

Blank maps, form DD, giving the location of the stations, may be 
obtained from the Bureau at $1.55 per thousand. Let the student write 
below the circle indicating each station upon the map the temperature 
at that station, and then proceed to draw the isotherms for each ten 
degrees. Draw first the isotherm of 30 , which passes through all sta- 
tions having a temperature of 30° and separates those having a higher 
temperature from those having a lower. Starting a little north of 
Boston, it runs westward north of Albany, south of Parry Sound, Sault 
Ste. Marie, and Marquette, east of St. Paul and Des Moines, through 
Concordia, Oklahoma, and El Paso, and thence passes northward east 
of Grand Junction. In a similar manner draw isotherms at ten-degree 
intervals from 70 to — 30 . 

Upon another blank map write the pressure at each station, and pro- 
ceed to draw the isobars for each tenth of an inch. First find the area 
of low pressure, which appears from inspection of the table to be the 
Lake region, with Grand Haven as a center. Inclose the center with 
the isobar of 29.60 inches, which passes south of Sault Ste. Marie, east 
of Dubuque and Davenport, north of Indianapolis, and west of Detroit. 
Locate the area of high pressure in Northwest Territory, and inclose it 
with the isobar of 30.70 inches. These will indicate the general pattern 
of the isobars. When the isobars are completed and numbered, draw 
upon the same map at each station a small arrow flying with the wind, 
as given in the table. Transfer the isotherms to the map of isobars. 
The two sets of lines may be drawn in different colors. Attach to each 
arrow the symbol used upon weather maps for clear, fair, cloudy, rain, 
or snow, as the case may be at each station ; or the area where the 
table indicates cloud, rain, or snow may be shaded lightly, and the areas 
where rain or snow is falling shaded more deeply. 

Observe upon the map thus drawn : (1) The pressure slopes between 
Grand Haven and Boston, Norfolk, Montgomery, Valentine, and Min- 
nedosa ; between Ou'Appelle and San Antonio ; between Swift Current 

410 






THE CONSTRUCTION OF A WEATHER MAP 411 



Observations taken at 8 A.M., 75th Meridian Time 





Si 






a 




00 




-a </i 
c jH 


2 








rt ■-" 










ni -1 






•3 




c s 


.2" 




-5 




c 6 


a, 




« 


V 


.9 c 


«j 






ij 


.2 a 


V 


Districts and 


M ■" 


~ 


0" u 


a. 


Districts and 


1 « 


3 


OJ C ^ 


a. 


Stations 


aJz 


rt 


.s.£§ 


-0 


Stations 


2-c 


rt 


Si £> 3 

— .- 


•a 








-O o_c 


c . 








T3 u_n 


c . 




s.s 


P. 


TJ-2 u 


« c 




S.S 


Q. 


•0.2 M 


a a 




^ 


E 


1^' 


in 




2 c 


s 

H 


c t> ij 


CO 


Atlantic Coast. 








Upper Miss. Val. 










Boston .... 


30.46 


34 


S E. 12 


cloudy 


Cairo .... 


29.88 


54 


S.W. 20 


clear 


Albany .... 


3°-34 


32 


S.E. 20 


" 


St. Louis . . . 


29 


84 


40 


w. 28 


cloudy 


New York . 


30.32 


33 


E. 34 


" 


Springfield, 111. . 


29 


74 


38 


W. ZO 


" 


Philadelphia . . 


30.24 


40 


S.E. 14 


rain 


Keokuk .... 


29 


84 


36 


w. 26 


" 


Washington 


30.16 


36 


N. Lt 


" 


Davenport . . , 


29 


62 


34 


W. 16 


rain 


Lynchburg . . . 


30.12 


36 


N.E. Lt. 


" 


Des Moines . . 


29 


96 


28 


N.W. 20 


cloudy 


Norfolk . " . 


30. 12 


42 


N. Lt. 


" 


Dubuque . . 


29 


64 


32 


N.W. 20 


snow 


Jacksonville 


30. 12 




s. 8 


clear 


St Paul .... 


29 


5:- 


24 


N.W. 16 


fair 


Tampa .... 


30.14 


>>3 


S.E. T2 


cloudy 


Missouri Valley. 










Gulf States. 










Kansas City . 


30.08 


32 


N.W. 12 


cloudy 


Atlanta .... 


30.02 


5° 


N.E. 6 


rain 


Springfield, Mo. . 


3° 


10 


28 


N.W, 26 


clear 


Mobile . . . 


30.06 


70 


s.w. Lt. 


fair 


Concordia . . . 


30 


30 


3° 


N.W. 24 


" 


Montgomery . 


29.98 


70 


S. 12 


cloudy 


Omaha .... 


30 


10 


24 


N.W. 16 


cloudy 


Vicksburg . . . 


30.08 


62 


N.W. 12 


fair 


Sioux City . 


30 


T2 


18 


N.W. 28 


" 


New Orleans . 


30.06 


70 


S.w. 6 


" 


Huron .... 


30 20 


10 


N.W. 30 


" 


Shreveport . . 


30.14 


54 


N.W. 8 


clear 


Bismarck 


3058 


- 4 


N W. 8 


clear 


Fort Smith . . . 


30.16 


;S 


W. 12 




Moorhead . . . 


30.30 


8 


N.W. 20 


cloudy 


Little Rock . . 
Galveston . . 


30.06 
30.06 


4 S 
66 


W. 12 

N.W. 6 


fair 

cloudy 


Northwest Ter. 










Palestine . . 


30.18 


52 


N.E. 6 


" 


Calgary .... 


30.62 


-16 


O 


clear 


San Antonio 


30.04 


62 


N. 14 


" 


Minnedosa . 


30.70 


— 12 


s.w. Lt 


" 


Fort Worth . . . 


30 24 


4 2 


N. 8 


fair 


Prince Albert . 


30.68 


—32 





fair 


Ohio Val. andTen. 










Swift Current . . 
Qu'Appelle . . 


30.72 
30.64 


— 12 
— 6 



Lt. 


cloudy 


Indianapolis . . 
Pittsburg . . . 


29.64 
29.88 


56 


S.W. 28 

s. 6 


clear 


Rocky Mt. Slopes. 










42 


rain 












Cincinnati . . . 


29 74 


58 


S. 14 


cloudy 


Havre .... 


30.42 


zero 


N.E. 8 


clear 


Columbus . 


29.72 


54 


S. 12 


" 


Helena .... 


3° 


40 


2 


w. Lt 


snow 


Louisville . 


29.76 


58 


S. 14 


" 


Miles City . . . 


3° 


5° 


2 


N. Lt 


fair 


Chattanooga . . 


29.98 


5° 


S.E. Lt. 


rain 


Rapid City . . . 


30 


5° 


4 


N.E. 6 


" 


Memphis . . 


30.02 


56 


w. 14 


fair 


Valentine . . . 


3° 


4 S 


4 


N.W. 12 


clear 


Nashville . . . 


29.92 


60 


w. 8 


cloudy 


North Platte . . 


3° 


48 


8 


N.W. 14 


" 


Parkersburg 


29.82 


5° 


S.E. 14 


" 


Cheyenne . 


30 


40 


8 


s. 8 


" 


Lake Region 










Lander .... 
Salt Lake City . 


30 
3° 


36 

00 


4 

40 


s.w. Lt 

S.E. 6 


cloudy 


Chicago .... 


29.54 


40 


S.W. 36 


cloudy 


Denver ... 


30 


38 


14 


N.E. 18 


clear 


Detroit .... 


29.64 


42 


S. IO 


" 


Pueblo .... 


3° 


3° 


iS 


E. Lt. 


fair 


Grand Haven . . 


29.50 


40 


S. 12 


" 


Santa Fe . . 


3° 


18 


24 





clear 


Marquette 


29.6S 


24 


W. 12 


snow 


El Paso .... 


3° 


14 


30 


N.E. Lt. 


" 


Sauk Sie. Marie 


29.62 


24 


E. 14 


" 


Abilene ... 


30 


26 


36 


N. 6 


" 


Duluth .... 


29.90 


26 


N.W. 18 


" 


Amarillo . . '. 


3° 


3° 


22 


N. 14 


" 


Cleveland 


29.68 


48 


S.E. 30 


rain 


Oklahoma . . . 


3° 


24 


30 


N. 14 




Buffalo .... 


29.78 


40 


S. 18 


cloudy 


Dodge City 


3° 


38 


18 


N.W. 12 




Parry Sound . . 


29.74 


28 


S.E. 36 


" 


Wichita . 


30 


3° 


24 


N.W. 14 




White River . . 


29.80 


18 


N. Lt. 


snow 


Grand Junction . 


30 


14 


32 


E. 20 


cloudy 



412 



APPENDIX IV 



and Salt Lake City. (2) The direction of the wind compared with that 
of the isobars and of the pressure slopes ; the general air movement in 
the cyclone. (3) The wind velocities near the center of low pressure; 
near the center of high pressure. (4) The course of the isotherms 
across the cyclone; across the anticyclone. (5) The position of the 
areas of cloud and of rain or snow in relation to the cyclone. 

Account for all these conditions. Make a forecast of the weather for 
March 15 and 16, 1899, at the place where you live. 

This exercise may be repeated as often as desired, by giving the 
students data obtained from other weather maps. Consult Ward's 
Practical Exercises in Elementary Meteorology. 

Daily weather maps may usually be obtained on request from any 
Weather Bureau Station. 

APPENDIX IV 1 



REFERENCE BOOKS 

The following list of standard books and periodicals is not intended 
to be a complete bibliography of the subject, but it comprises a large 
part of the literature in English, other than regular text-books on physi- 
cal geography, available for the student and teacher of that subject. As 
a rule, the best books are named first under each topic. See Hints to 
Teachers and Students on the Choice of Geographical Books, Mill, 
$1.25, Longmans, Green, & Co. 

General References 
The International Geography. $3.50. D. Appleton & Co., N.Y. 
Physiography, Huxley. $1.80. Macmillan. 
Our Earth and Its Story, Brown. 3 Vols., $9.75. Cassell & Co., 

N.Y. 

1 Abbreviations used in this appendix: A. G., American Geologist (Minneapo- 
lis) ; A. J. S., American Journal of Science (New Haven) ; B. A. G. S., Bulletin 
(Journal) of the American Geographical Society (N.Y.) ; B. G. S. A., Bulletin of 
the Geological Society of America (Rochester); G. J., Geographical Journal (Lon- 
don); G. S., Geological Survey (Washington) ; J. G., Journal of Geology (Chicago); 
J. S. G., Journal of School Geography (Lancaster, Pa.) ; N., Nature (London) ; 
N. G. M., National Geographic Magazine (Washington); N. G. Mon., National 
Geographic Monographs; P. S. M., Popular Science Monthly (N.Y.) ; S., Science 
(Lancaster, Pa.); S. G. M., Scottish Geographical Magazine (Edinburgh) ; S. R., 
Report of the Smithsonian Institution (Washington). 



REFERENCE BOOKS 413 

Outlines of the Earth's History, Shaler. $1.75. Appleton. 

Annual Reports, Monographs, and Bulletins of the United States 
Geological Survey. Apply to the Director, Washington, D.C. 
(Abbrev. G. S.) 

Reports of the Geological Surveys of the various states. 

Annual Report of the Smithsonian Institution. Apply to the Secre- 
tary, Washington, D.C. (Abbrev. S. R.) 

Periodicals 

National Geographic Magazine. $2.50. Washington, D.C. 

(Abbrev. N. G. M.) 
Bulletin (Journal) of the American Geographical Society. $4.00. 

N.Y. (Abbrev. B. A. G. S.) 

Geographical Journal, $6.00. London, Eng. (Abbrev. G. J.) 
Scottish Geographical Magazine. $5.00. Edinburgh, Scotland. 

(Abbrev. S. G. M.) 
Journal of School Geography. $1.00. Lancaster, Pa. (Abbrev. 

J. S.G.) 

Bulletin of the American Bureau of Geography. $1.00. Winona, 

Minn. 

Journal of Geology. $3.00. Chicago, 111. (Abbrev. J. G.) 
Bulletin of the Geological Society of America. $5.00. Rochester, 

N.Y. (Abbrev. B. G. S. A.) 

American Geologist. $3.50. Minneapolis, Minn. (Abbrev. A. G.) 
American Journal of Science. $6.00. New Haven, Conn. (Abbrev. 

A. J. S.) 

Science. $5.00. Lancaster, Pa. (Abbrev. S.) 

Nature. $6.00. London, Eng., and New York. (Abbrev. N.) 

Popular Science Monthly. $3.00. McClure, Phillips & Co., N.Y. 

(Abbrev. P. S. M.) 

Book I 

Chapter 1. — A New Astronomy, Todd. $1.30. Am. Book Co. 

Chapter 2. —Manual of Geology, Dana." $5.00. Am. Book Co. 
Text-Book of Geology, Dana. $1.40. Am. Book Co. 

Text-Book of Geology, Geikie. $7.50. Macmillan. 

Common Minerals and Rocks, Crosby. 40 cents. D. C. Heath & 
Co., Boston. 

Story of Our Planet, Bonney. 



414 APPENDIX IV 

Geological Studies, Winchell, $2.50. Scott, Foresman & Co., Chicago. 
N. 34, 400; 46, 348, 372; 59, 330. B. G. S. A. 11, 61. S. R. 
1896, 233. G. J. 13, 225. 

Book II 

The Earth and Its Story, Heilprin. $1.00. Silver, Burdett & Co., 
Boston. 

A Text Book of Geology, Brigham. $1.40. Appleton. 

Elements of Geology, Le Conte. $4.00. Appleton. 

Introduction to Geology, Scott. $1.90. Macmillan. 

National Geographic Monographs, 10 numbers, 20 cents each. Bound 
$2.50. Am. Book Co. (Abbrev. N. G. Mon.) 

Handbook of Physical Geology, Jukes-Browne. $175. Macmillan. 

Aspects of the Earth, Shaler. $2.50. Scribner's. 

Fragments of Earth Lore, Geikie. John Bartholomew, Edinburgh. 

Earth Sculpture, Geikie. $2.00. G. P. Putnam's Sons, N.Y. 

Scenery of Scotland, Geikie. $3.50. Macmillan. 

Any text-book of geology. 

Chapter 4. — 'Rocks, Rock Weathering, and Soils, Merrill. $4.00. 
Macmillan. 

Origin and Nature of Soils, Shaler. G. S. 12th Rep. 1, 219. 

Rivers of North America, Russell. $2.00. Putnam's. 

Geology of the Uinta Mountains, Powell, p. 181. Dept. of the 
Interior, Washington. 

Geology of the Henry Mountains, Gilbert, p. 93. Dept. of the In- 
terior, Washington. 

G. S. 14th Rep. 2, 149. 

Chapters. — Principles of Geology, Lyell, -I, 435. 2 vols. $800. 
Appleton. 

A. J. S. 116, 417; 152, 29. P. S. M. 25,594. Scribner's Monthly, 
22, 420. North American Review, 136, 212. Forum, 24,325. 

Chapter 6. — History of the Grand Canyon District, Dutton. G. S. 
Mon. II. $10.00. 

Canyons of the Colorado, Powell. $10.00. Flood & Vincent, Meade- 
ville, Pa. 

A. J. S. 112, 16, 85. P. S. M. 7, 385, 531, 670. 

Chapter 7. — Niagara Falls and Their History, Gilbert. N. G. Mon. 

B. G. S. A. 1, 66, 563 ; 9. 59. B. A. G. S. 31, 101. A. J. S. 128, 
123 ; 140, 425 ; 148, 455. S. R. 1890, 231. P. S. M. 49, 1. 



REFERENCE BOOKS 415 

Chapter 8. — Geology, Prestwich, I, 155. $6.25. Clarendon Press. 

Celebrated American Caverns, Hovey. $2.00. Robert Clarke Co. 
Cincinnati. 

Yellowstone National Park, Chittenden. $1.50. Robert Clarke Co. 

J. S. G., 1, 133. 

Chapter 9. — Illustrations of the Earth's Surface: Glaciers. Shaler 
and Davis. $10.00. Houghton, Mifflin & Co., Boston. 

Glaciers of the Alps, Tyndall. $2.50. Longmans, Green & Co., 
N.Y. 

Forms of Water, Tyndall. $1.50. Appleton. 

Glaciers of North America, Russell. $1.75. Ginn & Co. 

Ice Work, Past and Present, Bonney. $1.50. Appleton. 

First Crossing of Greenland, Nansen. $1.25. Longmans. 

Northward over the Great Ice, Peary. $6.50. Fred. A. Stokes, 
N.Y. 

Greenland Ice Fields, Wright. $2.00. Appleton. 

Glaciers of the United States, Russell. G. S. 5th Rep. 309. 

Second Expedition to Mt. St. Elias, Russell. G. S. 13th Rep. 2, 7. 
Glacier Bay and Its Glaciers, Reid. G. S. 16th Rep. 1, 415. B. G. 
S. A. 6, 199. J. G. 1, 219 ; 2, 649, 768 ; 3, 61, 198, 469, 565, 668, 833, 
875 ; 4, 582, 769, 912 ; 5, 229. A. J. S. 133. 1 ; 143, 169. P. S. M. 
29, 660; 46, 1. B. A. G. S. 28, 217. Century Magazine, 41, 865; 
44, 190; 50, 235. Cosmopolitan Magazine, 17, 296, 411. N. G. M. 
4, 19. 

Chapter 10. — Ice Age in North America, Wright. $5.00. Appleton. 

Man and the Glacial Period, Wright. $1.75. Appleton. 

The Great Ice Age, Geikie. $7.50. Appleton. 

The Canadian Ice Age, Dawson. $2.00. Scientific Pub. Co., N.Y. 

Studies in Indiana Geography, Dryer. $1.25. Inland Pub. Co., 
Terre Haute, Ind. 

Climate and Time, Croll. $2.50. Appleton. 

Cause of an Ice Age, Ball. 75 cents. Kegan Paul, Trench, Trlibner 
& Co., London. 

Island Life, Wallace. Chaps. VII-IX. $1.75. Macmillan. 

Terminal Moraine of the Second Glacial Epoch, Chamberlin. G. S. 
3d Rep. 291. 

Rock Scorings of the Great Ice Invasions, Chamberlin. G. S. 7th 
Rep. 147. 

The Glacial Gravels of Maine, Stone. G. S. Mon. XXXIV. 



416 APPENDIX IV 

The Illinois Glacial Lobe, Leverett. G. S. Mon. XXXVIII. 

Eighteenth Rep. Indiana State Geologist, 83. Indianapolis. 

J. G. 1, 52, 129, 246, 255; 2, 123, 517, 613, 708,837; 3, 70; 4, 129, 
948 ; 6, 147. B. G. S. A. 1, 287, 395 ; 4, 191 ; 7, 17, 31. B. A. G. S. 
30, 183, 217. S. R. 1893, 277. A. J. S. 128, 407 ; 135, 401. A. G. 
13, 397 ; 14, 12 ; 17, 16 ; 24, 93, 157, 205. 

Chapter ir. — Lakes of North America, Russell. $1.50. Ginn & Co. 

Present and Extinct Lakes of Nevada, Russell. N. G. Mon. 

Lake Bonneville, Gilbert. G. S. Mon. I. 

Lake Lahontan, Russell. G. S. Mon. XI; 3d Rep. 195. 

Glacial Lake Agassiz, Upham. G. S. Mon. XXV. 

Studies in Indiana Geography, Dryer. 

Mono Lake, Russell. G. S. 8th Rep. 1, 265. 

G. J. 1, 481 ; 6, 46, 135. S. G. M. 11, 60; 16, 193. B. A. G. S. 
25, 1 ; 30, 226; 31, 1, 101, 217. B. G. S. A. 1, 71, 297, 563 ; 2,243, 
465; 5,339; 7 > 327, 423- P. S. M. 45, 40, 224; 49,157. J.G.I, 
394. A. G. 14, 289; 18, 169. N. G. M. 8, 33, in, 233. A. J. S. 
133, 278 ; 140, 443 ; 141, 12, 201 ; 144, 290 ; 147, 105 ; 149, 1 ; 153, 
165. N. 43,203; 57,2ii. Forum, 5, 417. J. S. G. 1,65; 2,291. 

Chapter 12. — Rivers of North America, Russell. Chapter IX. 

Geology, Scott. Chap. XVIII. 

G. S. Mon. XXIII, in. N. G.M.I, 203. G. J. 5, 127. B. G. S.A. 
7,505. A. J. S. 112, 88. J. G. 4, 567, 657. S. 12, 131. 

Chapter 13. — Structure and Distribution of Coral Reefs, Darwin, 
$2.00. Appleton. 

Corals and Coral Islands, Dana. $5.00. Dodd, Mead & Co., N.Y. 

The Bermuda Islands, Heilprin. A. Heilprin, Philadelphia. 

Animal Life, Semper, p. 224. $2.00. Appleton. 

N. 22, 351 ; 35, 7 y ■ 37, 98, 393,414, 546; 39,236,424; 40, 53,203, 
222, 271 ; 41, 300; 42, 29, 162; 51, 203; 55, 373, 390. A. J. S. 130, 
89,169. G.J. 5, 73. P. S. M. 32, 241. 

Chapter 14. — Geology of the Uinta Mountains, Powell. 

Fragments of Earth Lore, Geikie, p. 36. 

The Northern Appalachians, Willis. N. G. Mon. 

The Southern Appalachians, Hayes. N. G. Mon. 

The Scenery of Switzerland, Lubbock. $1.50. Macmillan. 

Hours of Exercise in the Alps, Tyndall. $2.00. Appleton. 

The Alps from End to End, Conway. $5.00. Archibald Constable, 
London. 



REFERENCE BOOKS 417 

Mountaineering in the Sierra Nevada, King. 

Geology of Southern Oregon, Russell. G. S. 4th Rep. 

Mechanics of Appalachian Structure, Willis. G. S. 13th Rep. 2,217. 

Physiography of the Chattanooga District, Hayes. G. S. 19th Rep. 
2, 1. 

N. G. M. 6, 63. J. G. 4, 195. P. S. M. 39, 665. B. A. G. S. 29, 
16. A. J. S. 112, 414 ; 138, 257. 

Earthquakes, Milne. $1.75. Appleton. G. S. 9th Rep. 209. 

Fragments of Earth Lore, 36, 393. 

Island Life, Chap. VI. 

Popular Lectures and Addresses, Kelvin. Vol. II, 299. $2.00. 
Macmillan. 

S. R. 1892, 163. S. 5, 321. B. G. S. A. 1, 25; 2, 10; 4, 179; 6, 55. 
A. J. S. 104, 345, 460; 105,347,423; 106,6; 116, 95; 133, 102; 
144,177. N. 37,448; 46,224; 47, 81; 48, 551. J. G. 1, 543 ; 4, 177. 
P. S. M. 47, 362. G. S. 13th Rep. 274. 

Chapter 15. — Volcanoes of North America, Russell. $4.00. Mac- 
millan. 

Volcanoes, Judd. $2.00. Appleton. 

Volcanoes, Hull. $1.50. Scribner's. 

Volcanoes, Bonney. $2.00. Putnam's. 

Principles of Geology, Lyell. 

Mount Shasta, Diller. N. G. Mon. 

Characteristics of Volcanoes, Dana. $5.00. Dodd, Mead & Co. 

Geology of the Henry Mountains, Gilbert. 

Hawaiian Volcanoes, Dutton. G. S. 4th Rep. 80. 

Mt. Taylor and the Zuni Plateau, Dutton. G. S. 6th Rep. 105. 

Laccolite Mountains, Cross. G. S. 14th Rep. 2, 165. 

A. J. S. 108, 200 ; 127, 49 ; 133, 87, 433 ; 134, 19, 81, 349 ; 135, 15, 
213, 282 ; 136, 14, 81, 167 ; 145, 241 ; 148, 338. N. 34, 232, 343 ; 35, 
31 ; 36, 269, 297 ; 38, 299 ; 39, 279 ; 50, 483. P. S. M. 25, 363 ; 40, 
447. S. R. 1891, 163; 1892, 133, 153. S. G. M. 13, 246. G. J. 7, 
229. McClure's Magazine, 11, 17, 449. 

Chapter 16. — Earth Sculpture, Geikie. 

Scenery of Scotland, Geikie. 

The Scientific Study of Scenery, Marr. Methuen & Co., London. 

Physical Geography of Southern New England, Davis. N. G. Mon. 

G.S. 6th Rep. 225. Mon. XXII, 108. A. J. S. 112, 88. A. G. 23, 
207. B.G. S. A. 4, 133; 7, 377. B.A.G. S. 27, 161. 

DR. PHYS. GEOG. — 25 



41 8 APPENDIX IV 

Chapter 17. — Beaches and Tidal Marshes of the Atlantic Coast, 
Shaler. N. G. Mon. 

Sea and Land, Shaler. $2.50. Scribner's. 

Shore Line Topography, Gulliver, Proceedings American Acad, of 
Arts and Sciences, 34, 351. 

Features of Lake Shores, Gilbert. G. S. 5th Rep. 75. 

Natural History of Harbors, Shaler. G. S. 13th Rep. 2, 93. 

Salt Marshes, Shaler. G. S. 6th Rep. 

B. G. S. A. 7, 399. 

Book III 

The Depths of the Sea, Thomson. $7.50. Macmillan. 

The Atlantic, Thomson. 2 vols., $3.00. McDonough, Albany, N.Y. 

Three Cruises of the " Blake," Agassiz. 2 vols., $8.00. Houghton, 
Mifflin & Co. 

Thalassa, Wild. $3.00. Marcus Ward & Co., London. 

Deep Sea Soundings and Dredgings, Sigsbee. U. S. Coast Survey, 
Washington. 

Report of the Scientific Results of the Challenger Expedition, Narra- 
tive, Vol. I, and Summary, Vol. I. $20.00 per vol. Eyre & Spottis- 
woode, London. 

Deep Sea Exploration, Tanner. U. S. Fish Commission, Washington. 

Deep Sea Soundings in the North Pacific, Belknap. U. S. Hydro- 
graphic Office, Washington. 

Nature and Man, Carpenter, 316. $2.25. Appleton. 

G. J. 5, 360; 12, 113, 451 ; 14, 34, 426. N. 35, 33; 42, 357, 480; 
46, 348 ; 50, 377. S. R. 1890, 259 ; 1893, 545 ; 1894, 343. P. S. M. 
43, 39 ; 44, 334. S. G. M. 15, 505. Scribner's Mag. 12. 77. 

Chapter 21. — Climate and Time, Croll, p. 95. 

Popular Lectures and Addresses, Kelvin. Vol. Ill, $2.00. Macmillan. 

G. J. 4, 252. N. 40, 66; 50, 377. N. G. M. 5, 161. S. 2, 344. 
S. G. M. 13, 515. J. S. G. 2, 16, 122. S. R. 1891, 189. 

Book IV 

Elementary Meteorology, Waldo. $1.50. Am. Book Co. 
Elementary Meteorology, Davis. $2.50. Ginn & Co. 
Practical Exercises in Elementary Meteorology, Ward. $1.12. Ginn 
& Co. 

American Weather, Greely. Dodd, Mead & Co. 



REFERENCE BOOKS 419 

Meteorology, Russell. $4.00. Macmillan. 

Modern Meteorology, Waldo. $1.50. Sciibner's. 

Weather, Abercromby. $1.75. Appleton. 

Popular Treatise on the Winds, Ferrell. $4.00. John Wiley & 
Sons, N.Y. 

Bartholomew's Physical Atlas, Vol. Ill, Meteorology. $13.00. Archi- 
bald Constable, London. 

Illustrated Cloud Forms. $1.00. U. S. Hydrographic Office, Wash- 
ington. 

J. S. G. 1, 139. S. R. 1896, 125; 1897, 301, 317; 1891, 179. N. 
33,256; 37,469; 39,224; 40,330; 45,593; 47,2io; 48, 160; 53, 
136. P. S. M. 54, 89. 

Chapters 27 and 29. — Whirlwinds, Cyclones, and Tornadoes, Davis. 
50 cents. Lee and Shepard, Boston. 

The Law of Storms, Rosser. $1.25. Norie & Wilson, London. 

Aspects of the Earth, p. 197. 

N. G. M. 1, 40 ; 8, 65. S. 2, 711 ; 5, 45. P. S. M. 45, 138 ; 53, 307. 
A. J. S. 133, 453. N. 35, 91, 135; 38,104,149; 39,302; 53,589; 61, 
611. G. J. 2, 331. Scribner's Mag. 15, 229. 

Book V 

Plant Relations, Coulter. $1.10. Appleton. 

Minnesota Plant Life, Macmillan. Minn. Botanical Survey. St. Paul. 

The Great World's Farm, Gaye. $1.00. Macmillan. 

Geographical Distribution of Animals, Wallace. 2 vols., $10.00. 
Harper. 

Island Life, Wallace. Chaps. I.-VII. 

Geographical and Geological Distribution of Animals, Heilprin. 
$2.00. Appleton. 

Zoogeography, Beddard. $1.50. Macmillan. 

Lessons in the New Geography, Trotter. $1.00. D. C. Heath & Co., 
p. 160. 

Origin of Species, Darwin. $2.00. Appleton. 

Naturalist's Voyage, Darwin, Chap. XVII. $2.00. Appleton. 

Evolution, Le Ccnte. $1.50. Appleton. 

Factors in Organic Evolution, Jordan. $1.25. Ginn & Co. 

Life Zones and Crop Zones of the United States, Merriam. 10 cents. 
Dept. of Agriculture, Washington. 

N. G. M. 6, 229. N. 49, 302 ; 57, 213. J. S. G. 1, 97. 



420 



APPENDIX IV 



Chapter 32. — Ethnology, Keane. $2.60. Macmillan. 
Man, Past and Present, Keane. $3.00. Macmillan. 
Races and Peoples, Brinton. 
Races of Man, Peschel. $2.25. Appleton. 
Races of Man, Deniker. $1.50. Walter Scott, London. 
Anthropology, Tylor. $2.00. Appleton. 
Origin of Civilization, Lubbock. $5.00. Appleton. 
Dawn of History, Keary. $1.25. Scribner's. 
Some First Steps in Human Progress, Starr. $1.00. 
Vincent. 

Lessons in the New Geography, Trotter. 
P. S. M. 37, 577. 



Flood and 



INDEX 



PAGES 

Ablation 113 

Adaptation, of animals 366 

of plants 361, 362 

Agassiz, Lake 133, 146, 147 

Age, of mountains 190 

of rivers 155, 161, 162 

Air 2 73 _2 79 

composition of 273 

cooling of 282 

density of 276 

moisture in 280-282 

temperature of 276-279, 348 

visibility of 275 

weight of 275 

Air pressure 275, 276, 288 

distribution of 3 OI -3 r 1 

in a thunderstorm 326 

in a tornado 324 

in cyclones 313, 317, 320 

measurement of 275, 400, 401 

relation to temperature 302, 304 

Alaskan glaciers 117-119 

Alkali plains 173 

Alluvial cones and fans i7r 

Alluvial deposits 170, 171 

Alluvial lakes 149 

Alluvial plains 171, 172 

Alpine glaciers 109 

Alps, influence on man 225, 226 

structure of 187 

Amphitheaters 221 

Animal geography 364-382 

Animal realms and regions 367-379 

Animals, assist weathering 59 

domestication of 384 

earth habitable by 53-56 

food of 364 

lime deposits by 174, 175, 177 

on mountains. . . .,. 224, 225 

species of 365 

struggle for existence 364 



PAGES 

Antarctic drift 265, 266, 267 

Antarctic ice cap 120 

Antecedent stream 162 

Anthracite coal 36 

Anticline 179 

eroded 218, 186 

Anticyclones 306, 315-317 

Antitrade winds 307 

Antitrade zones, of climate 338-346 

Appalachian Mountains 184-187 

extent of folding in 192 

Aqueous rocks 33. 36 

Arctic-Alpine carpets 357 

Arctic Ocean 246 

Argon 273, 274 

Artesian wells 103 

Ashes, volcanic 198, 207 

Atlantic Ocean 245, 246 

currents in 265, 266 

Atmosphere 26, 273-348 

thickness of 273 

(see Air) 

Atolls 176 

Avalanche 113 



Bad Lands 212 

Banner cloud 283 

Bar 232 

Barograph 402, 403 

Barometer 275, 276, 400, 401 

Barometric gradient 288 

Barrier basins 147, 148 

Barrier beach 231 

Barriers, to animal migration 

, 365-367, 369, 37i, 372, 373 

to plant migration 360 

Basalt 35 

Base-level 79, 154 

Basin 61, 66 

Basins, of lakes 135-151 

Bay 228 



421 



422 



INDEX 



PAGES 

Bayou 77, 75 

Beach 231, 232 

Bed rock 31, 32 

joints in 222, 223 

Bends, or horseshoe curves 62, 75, 76 

Biosphere 26 

(see Life) 

Bituminous coal 33 

Black Hills 205 

Blizzard 315 

Block mountains 181, 182 

Block picture 53 

Bombs, volcanic 197 

Bonneville, Lake 136 

shore lines of 238 

Border seas 246, 245 

Bores 264 

Boulder clay no 

Boulders 30, 31, 122, 123 

Breaker 260 

Breccia 33 

Buttes 214, 212 

sea-made 231 

C 

Calderas 200 

Calms 307 

Canoe valley 184 

Canyons 84-91 

formation of 88, 89 

Capacity for vapor '. 281 

Capes 228 

Capture, by streams 157, 158 

Carbon dioxide 273, 274 

agent of weathering 57 

Caspian Sea 137 

Cataracts or falls 99-101, 153 

Caves or caverns 104, 105 

spouting 205 

Cayuga Lake 141, 142 

Celestial sphere 14 

Centrosphere 26, 27-29 

Chain of mountains 181 

Chalk 33 

Channel, of a stream 62 

Chasms 205 

Chimney, volcanic 197 

Cinders, volcanic 198 

Cirques 221 

Cirrus clouds 284 



PAGES 

Civilization 384, 385, 390 

Classification of land forms 242 

Clay 30, 59 

Cliffs 137, 138, 213, 223 

sea 230 

Climate 335-348 

effect on man 390 

in Eurasia 340 

in North America 341 

in the United States 341-346 

zones of 336 -340, 346 

Cloudbursts 326 

Clouds 282-284 

effect on temperature 278 

Coal 33. 36 

Coast forms 227 238 

Coast line, effect on man 391 

of United States, eastern 235-238 

Coast shelf 44, 172 

Coastal plain, Atlantic and Gulf 235 

Coasts, rising 227 

sinking 228 

Cold waves 315 

Colorado River system 81-91 

as source of knowledge 167 

Compressed folds 179 

Concretionary limestone 33 

Condensation 282 

Cone, volcanic 195, 197 

slope of 202 

Cones, alluvial 171 

Conglomerate 33 

Consequent streams 153, 162 

Continental block 39 

Continental climate ... 343, 344, 340 

Continental deposits, on sea floor 248 

Continental glaciers. 121, 124 

Continental islands 45, 228 

Contours 49 

Contraction theory 48 

Convection, atmospheric 287 

Coral reefs and islands 174-177 

Corrasion 65 

curve of 156 

Coulee lake 148 

Crater 195, 197 

Crater Lake 148, 149 

Crevasse, in a levee 77 

Crevasses, in glaciers no 

Culture, of man 383-385, 389 



INDEX 



423 



PAGES 

Cumulus clouds ■ 283 

Currents, alongshore. 231, 232 

Currents, ocean 264-270 

cause of '■ 267-269 

effect on isotherms 296, 297 

effects of 269, 270 

map of 256, 257 

Cusp 232 

Cut-off 76 

Cyclones 31 2-325 

causes of 320, 321, 322 

effect on rainfall 313, 320, 321, 333 

form of 323 

movement of air in 306, 312 

paths of 318,319 

tornadoes 323-325 

tropical 318-321 

D 

Day and night, length of 22, 23 

Dead Sea 137, 138 

Deeps 43, 40, 41 

Deflection of winds 289-292 

Degradation 66 

Degree, length of 25 

Delta 172, 234, 235 

of the Mississippi 78 

Density, of air 276 

of centrosphere 27 

of earth 27 

of sea water 255 

of water 255 

Denudation 216 

Deposition or sedimentation 168-177 

by animals and plants 174-177 

by evaporation 173, 150 

by glaciers 170, 115-117, 126-131 

by springs 106 

by streams 170-172 

by winds 168, 169 

Deposits on sea floor, 248, 249 

Depression , area of 39, 43 

Depression or subsidence 47, 48, 190-193 

Dew 285, 286 

Dew-point 282 

Diastrophic basins 135-138 

Diastrophism 47 

Dikes 203, 204, 205 

Dissected plateau 217 

Distributary 78 



PAGES 

Divides 61 

development of 155, 156 

migration of 157, 158 

Domestication 384, 385 

Drainage 163 

Drainage systems, development of .. .152-163 

Drift 60 

glacial 114, 115-117, 122 134 

shore 231, 232 

Drift plain 132, 131 

Drouth plants 357 

Drowned valleys 95 

Drumlins 129, 130 

Dunes 169 

Dust deposits 207 

E 

Earth, attitude of 20-22, 53 

axis of 17, 20 

curvature of 10 

density of 27 

face of 38-48 

orbit of 18, 23 

Plan of 39-44, 55 

revolution of 1 8, 54 

rotation of 17, 54 

shape of 10-12 

size of 12, 54 

structure of 26-37 

surface of 38-46 

temperature of 28, 53 

why habitable 53 - 56 

Earth-crust 26, 29-37 

acted on by ground-water 102-107 

acted on by internal and external forces 

239-242 

acted on by streams 63 

density of 27 

movements in 47, 48, 199-193 

structure of 37, 29 -34 

Earthquakes 190, 191 

East coast climate, in northern antitrade 

zone 340 

Ebb tide 261 

Edentates 370 

Elevation, area of 39. 43 

Elevation or upheaval 47, 48, 190-193 

effect on streams 160, 90 

Ellipse 18 

Equatorial calms 307 



424 



INDEX 



PAGES 

Equatorial currents 265, 266 

Equatorial rains 327, 329 

Equinoxes 23, 15 note 

Erosion 66, 57-67 

effects on land surface 212-224 

of mountains 182-190, 218-220 

Erratics 123 

Eruptions, volcanic 195-203 

Eruptive rock 34 

Escarpment 214, 97 

Eskers 129 

Estuary 228 

Evaporation 280, 281 

Eye of a storm 320 

F 

Falls, or cataracts 99-101, 153 

Fan fold 179 

Fans, alluvial 171 

Fault 178 

Faulting, cause of 191-193 

Ferrel's Law 290 

Filled valley 161 

Finger Lakes 140-142 

Fiords 229, 133 

Floe 270 

Floebergs 270 

Flood plains 62, 73-79, 154 

and man 164 

Floods 76, 77 

Flow, of tide 260 

Focus, of an earthquake 190 

Fog 282 

Folded mountains 182-187 

Folding, cause of 191-193 

Folds, rock 1 78, 1 79 

Food, of animals 364 

of man 384, 385 

of plants 350 

Foreland 232 

Forests 357-359. 35 2 "354 

effect on drainage 163 

Fossils 365, 369 

Frost 285, 286 

agent of weathering 57 

G 

Geocentric theory 17 

Geography 27 

Geysers 106 



PAGES 

Glacial drift 114, 115-117, 122-134 

Glacial sculpture 220, 221 

Glaciated basins 138 147 

Glaciation 115, 220, 221 

effect on streams 160, 161, 132, 133, 145 

fiords caused by , 229 

of Europe 133, 134 

of North America 130-133 

Glaciers 109, 108-121 

abrasion by .... 115 

effect on animals 367 

effect on plants 363 

effect on valleys 220 

formation of 108 

forms of deposits by 115-117, 126-131 

melting of 113, 112 

movement of 109-112, 120 

Gneiss 35 

Gobi, desert, lakes in 136 

Gorges 153, 97, 101 

(see Canyons) 

Graded plain 234 

Granite 35,58 

Gravel 30, 65 

Gravity, agent of land sculpture 210 

assists weathering 58 

Great Basin 135, 136, 138 

mountains in 181, 182 

Great Lakes 92, 93, 143-146, 267 

Great Rift Valley 138 

Great Salt Lake 136 

Green River 81-85 

Greenland ice cap 119 

Ground swell 259 

Ground-water 102-107 

Gulf Stream 265, 266 

H 

Habitable region 349 

Hachures 51 

Hailstones 285 

Hanging valleys 221 

Hawaiian volcanoes 198-202 

Heat (see Temperature) 

Heredity, law of 361 , 364 

Hoarfrost 286 

Hogbacks 206 

Hook 232 

Horseshoe curves, or bends 62, 75, 76 

Horseshoe lakes 76 



INDEX 



425 



PAGES 

Humidity 281, 282 

controls distribution of plants 354 _ 359 

measurement of 282, 401, 402 

Humus 30, 163 

Hurricanes 319-322 

Hydrographic basin 62 

Hydrosphere 26 

(see Sea) 
Hygrometer 282, 401, 402 

I 
Ice 235, 270 

(see Glaciers) 
Icebergs 120, 118, 271 

transportation by 170, 271 

Ice-dammed lakes 145, 146 

Igneous rocks 34-36 

Indian Ocean 246 

currents in 266 

Inland seas 246 

Insolation 293 

Intermediate plants 357~359 

Intermorainic lakes 148 

Intrusive rock 35 

Irrigation 164 

Islands 45, 228 

animals on 378 

Isobars 288 

Isostasy 47, 48 

Isotherms 294 

J 

Jetties 78 

Joints, in bed rock 222, 223 

Jura Mountains 183 

K 

Karnes 129 

Kettle holes 129, 128, 139 

Kilauea 200 

Knobs 128, 220 

Krakatoa 198 

Kurosiwo 265 

L 

Labrador current 266 

Laccolites 205 

Lacustrine plains 172 

Lagoon, in an atoll 176 

Lagoons, shore 148, 231 



PAGES 

Lahontan, Lake 136 

Lakes, and lake basins 135-151 

destruction of 150 

effects on a river 93, 101, 150 

Great 143 

horseshoe or crescent-shaped 76 

ice-dammed 145, 146 

mountain 142 

relation to rainfall and drainage 150 

Land, height of 45, 40, 41 

once covered by the sea 47 

surface of 44 

Land forms, classified 242 

Land sculpture 210-224 

Land surface, effect of erosion 212-224 

temperature of 53, 294, 296, 300 

Landscapes 210 

Landslide 168, 191 

Lapilli 198 

Latitude 23, 24 

Laurentian Lakes 143-146 

Lava 195, 197-202 

Lava flow3 203, 204 

Levees 77 

Life 349"39 2 

(see Plants and Animals) 

Lime, deposits of *73 -1 77 

in sea water - 250 

Limestone 33 

formation of 176, 177 

ground-water in 102-105 

Lithosphere 26 

(see Earth-crust) 

Llanos 357 

Loam 30 

Loch Katrine 143 

Loess 124, 169 

Longitude 24, 25 

Lost rocks 123 

M 

Malaspina Glacier 119 

Man, ascent of 383-385 

earth as home of 53-56 

food of 384, 385 

geography of 383~39 2 

influence of mountains 224-226 

influence of streams 163-167 

influence of the sea 27T, 272 

races of 386-389 



426 



INDEX 



PAGES 

Mantle rock ; 29, 30 

formation of 59, 6° 

removal and deposition of 168-177 

Maps 48-51, S95-39 8 

Marl 30 

Marsh plants 355, 357 

.Marshes 132, 163 

tidal 231 

Marsupials 368 

Mature stream 154, 162 

Mauna Loa 198-202 

Meanders, or bends 62, 75, 76 

development of 158 

Mechanical cooling of air 282 

Meridians 24, 25 

Mesas 214, 212 

Metamorphic rocks 35, 36 

Mica schist 35 

Migration, of animals 365-367, 376, 378 

of men 386, 387 

of plants 359, 360 

Mississippi River system 6S-80 

deposits by 172, 173 

Missouri River 69-71 

Models 51, 52, 393-395 

Moisture in the air 2S0-286 

Monotremes 368 

Monsoons 307 

Moon, causes tides 261-263 

Moraines 113-115, 116 

in North America 126-128 

Moulin 113 

" Mound and sag " surface 128 

Mountain climate 347, 348 

Mountain lakes 142 

Mountain range 180, 181 

Mountains 180, 190, 178-226 

age of 190 

block 181, 182 

complexly folded 183-187 

erosion of 182-190, 218-220 

formation of 191-193 

influence on life 224-226, 391 

plateau 189 

relict 188, 189 

simply folded , 182 

volcanoes 194-209 

Muck 30 

Mud rock 32 

Muir glacier 117, 118 



N 

PAGES 

Natural bridge 105 

Natural resources 391, 392 

Neap tide 263 

Neck, volcanic 207 

Nev£ 108 

Newer drift 126 

Niagara Falls 98-100, 166 

Niagara Gorge 97-100 

Niagara River 95-101 

Night and day, length of 22, 23 

Nile River 164 

Nimbus clouds 284 

Nitrogen 273, 274 

Noah's brush heap or barnyard 124 

O 

Ocean basins 24 5-247 

Ocean currents 264-270 

cause of 267-269 

effect on temperature 269, 270, 296, 297 

map of 256, 257 

Oceanic climate 340 

Oceanic islands 45 

Oceanography 244 

Ohio River 69, 73 

Old stream 162, 155 

Older drift 124 

Oolitic limestone 33 

Ooze 248, 249 

Orbit, of earth 18, 23 

Organic deposits, on sea floor 248, 249 

Outcrop 32 

Oxbow or horseshoe-shaped lakes 76 

Oxygen 273, 274 

agent of weathering 57 

P 

Pacific Ocean 245 

currents in 265 

Pack 270 

Pampas 357 

Parallels 24 

Passes, at mouth of Mississippi 78 

Passes, in mountains 187, 219 

Peaks 187, 219, 128 

Peat 30 

Pebbles • 30 

Peneplain 218 

Peninsulas 228 



INDEX 



427 



PAGES 

Physiographic cycle 242, 241 

Pipe, volcanic 197 

Piracy, by streams 157, 158 

Plain, alluvial .' -- 172 

coastal -152, 235 

drift 132, 131 

graded 234 

lacustrine 172 

peneplain 218 

Plant geography 349-363 

in the animal regions. .368, 371, 372, 374, 376 

Plant societies 354 

Plants, adaptation of 361 

assist weathering '. 59 

control by humidity 354 _ 359 

control by temperature 35 I_ 354 

domestication of 385 

earth habitable by 53-56 

food of 350 

lime deposits by 174, 177 

migration of 359, 360 

on mountains 224, 225 

species of 362 

struggle for existence 360-362 

Plateau 152, 178 

Plateau mountains 189 

Plateaus, sculpture of 212-218 

Playas 136 

Plug, volcanic 206 

Polar regions, climate of 346 

Polar whirls 307-3 1 1 

Polaris, or Polestar 13.14 

Population of world 387, 388 

Prairies 357 

Precipitation 284, 285 

(see Rainfall) 

Pressure in centrosphere 27 

Pressure of atmosphere 275, 276, 288 

distribution of 301-31 1 

in a thunderstorm 326 

in a tornado 324 

in cyclones 313, 317, 326 

measurement of 275, 400, 401 

relation to temperature 302, 304 

Pressure of sea water 255 

Pressure slope 288 

Prevailing westerlies 307 

Profiles 52, 53 

Promontories 228 

Psychosphere 26 



PAGES 

Pudding stone 33 

Pumice 198 

Purgatories 205 

Pyramid Lake 136 

R 

Races, caused by tides 264 

Races, of men 386-389 

Rain gauge.. 285, 402 

Rainfall 3 2 7-334 

agent of land sculpture 210, 222 

agent of weathering ■ 57 

caused by cyclones 313, 320, 321, 333 

causes of 327 

factor in climate 335 _ 347 

in United States 343-346 

measurement of 2S5, 402 

Rain-wash, curve of 156 

Range, of mountains 180, 181 

Range of temperature 298-300 

Rapids 153 

Red clay, on sea floor 249 

Reefs , 174-177 

Regelation in 

Rejuvenated stream 161 

Relict mountains 188, 189 

Relief 44 

causes of 46-48 

range of 46 

representation of 48-53 

Residual soil 60, 168 

Revolution, of earth 18 

Ridge, of mountains 180 

Rift basins 137, 138 

Rivers, action of 61-67 

blocked 235 

falls in 99-101, 153 

life history of 152-154 

three typical 68-101 

(see Streams) 

Rock sphere 26 

(see Earth-crust) 

Rocks, classified 36, 29 35 

formation of sedimentary 177 

(see Weathering, Erosion, Sculpture, 
Glaciation) 

Rotation, of earth 17 

effect on winds 289-292 

Run-off 61, 62 

of mantle rock 168 



428 



INDEX 



s 

PAGES 

St. Lawrence River system. . .92-101, 143-146 

Salt, deposits of 173, 174 

in sea water 250 

Sand 30 

Sand bars 62, 63 

Sandstone 32, 33 

formation of 177 

Saturation 280 

Scoriae 198 

Sculpture, land 210-224 

coast 230 

Sculptured forms, development of . . . . 216-218 

Sea 243-272 

action on coast lines 227-238 

currents in 231, 232, 264-270 

depth of 43, 40, 41 

figure of 243-247 

influence on man 271-272 

1'fe in 379-382 

movements of 258-271, 254 

surface of 244, 245 

temperature of 251-254 

tides in 260-264 

volume of 46, 243 

waves in 258-260 

Sea cliff 230 

Sea floor, or bottom 247-249 

study of 244 

temperature of 251-254 

Sea ice 270, 271 

density of 255 

Sea level 244, 245 

Sea water 250-255 

composition of 250 

pressure and density of 255 

Seasons 19-23 

in different climatic zones. .339, 337, 338, 346 

Sediment, in streams 63-65 

Sedimentary or aqueous rocks 33, 36 

formation of 177 

Sedimentation, forms of 168-177 

Shale 32 

formation of 177 

Shore drift 231, 232 

Sills 205 

Sinkholes 104, 105 

Skerries 230 

Slate 32 

Snow 285 



PAGES 

Snow line 108 

Soapstone 32 

Soil 29, 30 

Solstices 23, 15 note 

Solution 1 73 

basins formed by 144 

caves formed by 104 

deposits from 173 177, 105, 106 

rock carried in 64 

Southern Ocean 247 

Spit 232 

Spouts 325 

Spring tide 263 

Springs 102, 103 

hot 106 

mineral 105, 106 

Spurs 219 

Stacks 231 

Stalactites 105 

Stalagmites 105 

Stars, apparent movement of 13, 14, 19 

Steppes 357 

Stereogram 53 

Storm paths 318 

Storms 312-326 

Strata 32 

formation of 171 

Stratification 171 

Stratified rocks 34 

formation of 177 

Stratus clouds 284 

Streams, action of 61-67 

agents of land sculpture 211-219, 224 

classified 161, 162 

corrasion by 65 

development of 152-163 

effect of elevation 160, 90 

effect of glaciation. . .160, 161, 132, 133, 145 

effect of lakes 93, 101, 150 

effect of subsidence 160, 95 

influence on man 163-167 

transportation by 63-65 

underground 103 

(see Rivers) 

Striae 115 

Stromboli 194, 195 

Struggle for existence 360-362, 364 

Subsequent streams 159, 160, 162 

Subsidence, or depression 47, 48, 190-193 

effect on streams 160, 95 



INDEX 



429 



PAGES 

Sun, apparent movement of I 5 -I 7. 22 

causes tides 263 

heat from 19, 23, 293 

size of 18 

Superimposed stream 162 

Superior, Lake 143-146 

Survival of the fittest 362, 364, 366 

Suspension, rock carried in 64 

Swamp plants 355. 357 

Syenite 35 

Syncline 179 

eroded 218, 185 

System, of mountains 181 

T 

Talus 60, 168 

Temperature, agent of land sculpture. 210, 218 

agent of weathering 58 

controls distribution of plants 35 I_ 354 

distribution of 294-300 

effect of clouds 278 

effect of elevation 347, 348 

factor in climate 335~348 

in United States 343"346 

influence of ocean currents 

269, 270, 296, 297 

measurement of 279, 398, 399 

of air 276-279, 348 

of centrosphere 28 

of the sea 251-254 

range of 298-300 

zones of 297, 298 

Terrace, wave-built 23 1 , 232 

wave-cut 230 

Terraces 84, 85, 160, 161 

Terrane 216 

Thermal equator 297 

Thermograph 402, 403 

Thermometer 279, 398, 399 

Throw i7g 

Thunderstorms 325, 326 

Tidal marsh 231 

Tides 260-264 

effect on coast lines 235 

Till 116 

Tilted blocks 181 

Tornadoes 323-325 

Towers 212, 213 

Trade winds- 306 

Trade zone, of climate 336, 337 



PAGES 

Transportation, by glaciers n 3-1 15 

by icebergs 170, 271 

by streams 63-65 

by winds 168, 169 

Trap rock 35 

Trellised drainage 160, 185 

Tributaries 61 

Tropical calms 307 

Tropical cyclones 318-322 

Tropical rains 332 

Tufa 33 

Tundras 357 

Typhoons 319-322 

U 

Uinta Mountains 182, 183 

Underground waters 102-107 

Undertow 230 

Upheaval, or elevation 47, 48, 190-193 

V 

Valleys, development of 155 

drowned 95 

effect of glaciers on 220 

filled 161 

formation of 65-67 

hanging 221 

in mountains 219 

Vane, wind 403 

Vapor 280-282 

quantity in air 273, 275, 280 

Variable zone 227 

Variation, law of 361 , 364 

Vegetation (see Plants) 

Vesuvius 196, 197 

Volcanic basins, of lakes 148, 149 

Volcanic cone 195, 197 

slope of 202 

Volcanic rock 34 

Volcanoes 194-209 

causes of 208, 29 

distribution of 207 

W 

Warm waves 315 

Water gap 160 

Water plants 355 

Water sphere 26 

(see Sea) 
Water surface, temperature of.. .294, 296, 300 



430 



INDEX 



PAGES 

Waterspout 325 

Waves 258-260 

erosive action of 230, 231, 232, 260 

Waves of temperature 315 

Weather 335 -348 

in the United States 341-346 

Weather maps 318, 314, 316 

construction of 410-412 

Weather observations 279 

Weathering 57-60 

differential 223 

Wells ro2, 103 

West coast climate, in northern anti- 
trade zone 340 

Wind deposits 168, 160 

Wind gaps 160 

Wind sculpture 221, 222 



PAGES 

Wind velocity 289 

in tropical cyclones ■ . . . 319, 320 

in tornadoes 323, 324 

measurement of 404 

Winds 287 

cause of ocean currents 267-269 

deflection of 289-292 

distribution of 304 311 

effect on isotherms 296, 297 

erosive effect of 59 

factors in climate 335~34° 

in storms 312-326 

relation to pressure 288, 304-311 

Y 

Yellowstone River 70, 71 

Young stream 161 



■h 



